          United States Court of Appeals
                     For the First Circuit


No. 16-2275

                       GRANDVILL D. LAWES,

                      Plaintiff, Appellant,

                               v.

                CSA ARCHITECTS and ENGINEERS LLP,

                      Defendant, Appellee,

  PUERTO RICO PORTS AUTHORITY; MAPFRE-PRAICO INSURANCE COMPANY;
    MUNICIPALITY OF SAN JUAN; CONSTRUCTORA SANTIAGO II, CORP.;
 RAFAELA RIVIERE-ANDINO; MIGUEL A. BONILLA, INC.; COOPERATIVA DE
   SEGUROS MULTIPLES DE PUERTO RICO; PUERTO RICO ELECTRIC POWER
    AUTHORITY; ACE INSURANCE CO.; INTEGRAND ASSURANCE COMPANY;
          Q.B. CONSTRUCTION SE; TRIPLE-S PROPIEDAD, INC.

                           Defendants.


          APPEAL FROM THE UNITED STATES DISTRICT COURT
                 FOR THE DISTRICT OF PUERTO RICO

         [Hon. Daniel R. Domínguez, U.S. District Judge]


                             Before

                 Torruella, Lipez, and Thompson,
                         Circuit Judges.


     Jorge M. Izquierdo San Miguel, with whom Izquierdo San Miguel
Law Office, P.S.C. was on brief, for appellant.
     Ricardo F. Casellas-Sánchez, with whom Diana Pérez-Seda,
Casellas Alcover & Burgos P.S.C., Fernando J. Gierbolini-González,
Richard J. Schell, and Monserrate Simonet & Gierbolini, LLC, were
on brief, for appellee.
June 18, 2020
            THOMPSON,   Circuit    Judge.      This   case   involves    a

construction   project,   a   pedestrian-involved     collision,   and   a

twelve-day Daubert hearing that culminated in the exclusion of

plaintiff's only expert witness pursuant to Federal Rule of Civil

Procedure 26 and Federal Rule of Evidence 702.          With his expert

ousted, plaintiff's negligence case collapsed halfway through

trial, and then the district court entered judgment as a matter of

law for defendants.       The plaintiff has appealed the entry of

judgment against him and the district court's evidentiary rulings,

which sounded the death knell for his suit under Article 1802 of

the Puerto Rico Civil Code.       On this voluminous record, even from

our deferential perch, we find that the district court erred.

So we vacate the lawsuit's dismissal and remand the matter for

proceedings consistent with this opinion.

                               BACKGROUND

            In 2011, plaintiff-appellant Grandvill Lawes was hit by

an SUV while walking in a construction-affected area near Old San

Juan, Puerto Rico.      The facts are drawn from a massive record,

including myriad motions and depositions, a 188-page pretrial

order, and several weeks of trial.1        We therefore beg the reader's

patience as we set the scene and describe the litigation that

followed.


     1 All docket references ("D. _") are to Lawes v. Q.B. Constr.,
No. CV 12-01473 (D.P.R.).


                                   - 3 -
                                    The Scene

             Fernández Juncos Avenue ("Fernández Juncos" for short)

is an undivided four-lane highway, with two eastbound lanes and

two westbound lanes of traffic.                  Calle Del Tren, a two-lane

arterial roadway, lies to the north of Fernández Juncos.                  These

parallel roadways are separated by a cement median.                   Fernández

Juncos runs alongside San Juan Bay, and connects the working

waterfront (particularly, for our purposes, Piers 8, 9, and 10) to

Old San Juan, Puerto Rico.           There is a sidewalk adjacent to the

waterfront       to   the   south   of    Fernández   Juncos   (the   "southern

sidewalk"), and there is a sidewalk to the north of Calle Del Tren

(the "northern sidewalk").               Before construction, pedestrians,

including sailors whose ships are docked at the waterfront piers,

could use the sidewalks on either side of the combined roadways to

travel into Old San Juan.                Using just the southern sidewalk,

pedestrians heading into town could walk to the very end of the

piers before needing to cross over.2

             Around 2010, the Bahía Urbana Pier 7 and 8 Improvement

Project,     a    government-funded        construction   project     meant   to

beautify the waterfront just outside of Old San Juan, was initiated

and, thereafter, significantly changed the landscape of the area.


     2 Certain sailors testified at trial that they used the
southern sidewalk in order to avoid an area along the northern
sidewalk known as the "hot corner," where drug users purportedly
loiter and harass passersby.


                                         - 4 -
Defendant CSA Architects and Engineers, LLP was hired to draw the

plans for the Project.               CSA was also responsible for designing a

Management of Traffic plan ("MOT") to safely control vehicular and

pedestrian traffic in the construction-affected area.                        Defendant

Q.B. Construction, the Project's primary contractor, was tasked

with implementing CSA's designs, including the MOT.                    As instructed

by CSA's MOT, Q.B. installed a temporary concrete barrier along

the southern sidewalk near the middle of the block (the "midblock

barrier").           The midblock barrier closed part of the southern

sidewalk -- but only part -- from pedestrian use.                      The midblock

barrier also jutted into Fernández Junco's southernmost eastbound

lane of traffic, reducing the width of that lane.

               According to defendants (and as designed in the MOT),

Spanish-language signs at a permanent, mechanical crosswalk near

Pier 9 indicated that the southern sidewalk was partially closed

and instructed pedestrians to cross over to the unobstructed

northern sidewalk.3            If pedestrians didn't spot the signs, didn't

understand them, or chose to ignore them, nothing prevented them

from       walking    along    the     southern    sidewalk    until   the    concrete

midblock       barrier,       nearly    300   feet   away     from   the   crosswalk.



       3
       Lawes disputes that certain Spanish-language signs provided
for in the MOT (including a sign that was supposed to instruct
pedestrians to cross over to the northern sidewalk at a crosswalk
near Pier 9) were in place at the time of Lawes' accident.



                                           - 5 -
At that point, they could either walk back to the crosswalk (about

3/4 the length of a football field) and risk walking toward the

hot corner or they could jaywalk.4

                                The Accident

            Lawes was one of several merchant marines docked at San

Juan Bay on October 22, 2011, when he and his shipmate, Carlos

Gordon, ventured off their ship to grab dinner in Old San Juan.

Gordon, who had visited the area prior to construction, normally

traveled into town using the southern sidewalk.             When Gordon and

Lawes reached the recently implemented midblock barrier on the day

of the accident, however, they took a detour:               they jaywalked

across Fernández Juncos and Calle Del Tren, and resumed their trek

along the northern sidewalk (a healthy distance away from the "hot

corner").   When the sun had set and the street was dark, the pair

journeyed   back    to   the   piers.     They   started   on   the   northern

sidewalk, which would have led them to a permanent crosswalk back

to the piers.      They decided to jaywalk a second time.

            Lawes took the lead.        After successfully crossing Calle

Del Tren, he attempted to cross Fernández Juncos.           He was standing

on the roadway's yellow divider, two eastbound lanes away from the

southern sidewalk, when something awful happened: a traffic light


     4 Apparently, some pedestrians came up with a third solution:
scurry alongside the midblock barrier, against oncoming traffic,
without crossing Fernández Juncos in the construction-affected
area.


                                    - 6 -
changed down the block and cars began rushing toward Lawes from

both    directions,   trapping     him    on   the     yellow   divider.

Seconds later, he was struck head on by a westbound SUV.        Lawes is

now quadriplegic and will need medical care for the rest of his

life.   As he tells it, his medical expenses have already reached

$10 million.

                             The Lawsuit

            On June 14, 2012, Lawes filed a negligence-based lawsuit

under Article 1802 of the Puerto Rico Civil Code against public

and private entities involved in the Bahía Urbana construction

project, including the Project's contractor Q.B.; the Constructora

Santiago Corp. II (another construction company involved in the

Project); the Puerto Rico Ports Authority; the Municipality of San

Juan; and their respective insurance companies.        Soon after, Q.B.

filed third-party complaints against Rafaela Riviere-Andino, the

driver who struck Lawes with her car, and her insurer. On November

26, 2013, Q.B. and Riviere-Andino jointly filed a third-party

complaint against: CSA, the Project's designer; Miguel A. Bonilla,

Inc., the Project's inspection firm; the Puerto Rico Electric Power

Authority    ("PREPA"),   which    was    conducting    maintenance   on

streetlights in the construction-affected area; and these parties'




                                  - 7 -
insurance companies.5       Thereafter, Lawes amended his complaint

twice, adding Q.B.'s insurers and CSA as direct defendants in 2014

and 2015, respectively.         Although CSA is the only remaining

defendant on appeal,6 we are tasked with reviewing district court

rulings that resolved concomitant arguments raised by CSA, former

defendant    Q.B.,   and   certain    third-party        defendants   mentioned

above.    Thus, to aid the reader in understanding the district

court's reasoning and our review of it, we must occasionally

provide facts and conduct analysis concerning parties who are no

longer part of the litigation.

             After instituting his lawsuit, Lawes did what any smart

plaintiff in his position would do:          he retained an expert witness

to   opine   on   the   standard     of    care   owed    to    pedestrians   in

construction-affected      areas     and     to   explain      how   defendants'

negligence caused his accident.           Enter scene:     Ralph Aronberg, the


      5To simplify our recitation of the various defendants and
third-party defendants in this litigation, we are omitting the
names of the insurance carriers and will refer only to the parties
they insured. Over the course of the litigation, there were no
meaningful differences between the theories advanced by the
insurance carriers and the insured, and (in some instances) the
insurers and the insured were represented by the same counsel.
      6Lawes voluntarily dismissed his claims against Constructora
Santiago Corp. II and the Ports Authority in August 2012 and
January 2013, respectively.    The district court entered partial
judgment for the Municipality of San Juan and its insurance
carrier, dismissing both Lawes' claims and the third-party
complaint against the Municipality. And on April 19, 2017, this
Court granted Lawes' unopposed motion to dismiss all remaining
appellees except CSA, following a partial settlement.


                                     - 8 -
traffic engineer that Lawes brought into the case as his star (and

only) expert witness. For the reasons we explain later, Aronberg's

expert opinions were crucial to Lawes' case, and the district

court's exclusion of them is part of the reason why Lawes appealed.

Thus, Aronberg (and his opinions) are the primary focus of our

review.   That's why we're going to walk you through his role in

the case, starting with his first act (here, a preliminary report).

          Aronberg's Preliminary (and Only) Expert Report

              On January 25, 2013, about six months after Lawes filed

his   first    complaint,   Aronberg   submitted   a    three-page,   self-

described "preliminary report."        (Spoiler alert:      this would be

the only expert report Aronberg produced.) Attached to the report,

Aronberg provided a copy of his CV, a list of recent trials and

depositions in which he testified as an expert, and excerpts from

the 2009 edition of the Manual on Uniform Traffic Control Devices

("MUTCD") (which we'll discuss often here).            Aronberg, according

to his CV, is an expert in traffic accident reconstruction, traffic

engineering design, work-zone traffic control evaluation, and

pedestrian safety.     A 1978 graduate of the University of Virginia

with a Bachelor of Science in Civil Engineering, he also holds a

Master of Science in Engineering Management from Nova Southeastern

University in Fort Lauderdale, Florida.            Aronberg worked for

several years as a traffic engineer in Florida before founding a

consulting firm in 1983.      In the four years before he was hired by


                                  - 9 -
Lawes, Aronberg qualified to serve as an expert witness in sixteen

trials and was deposed as an expert over forty times.

            The report begins by disclosing the source of Aronberg's

methodology:    the MUTCD,7 a set of guidelines published by the U.S.

Department of Transportation, which Aronberg later described as

the "Bible" for traffic engineers.            The MUTCD (among other things)

includes      guidance     on       "[p]edestrian       [c]onsiderations"      in

construction zones.        It advises, for example, that "pedestrians

need a clearly delineated and usable travel path" in temporary

traffic control ("TTC") zones.           Because "pedestrians are reluctant

to retrace their steps" or "add distance," they should be provided

a "convenient and accessible path" that replicates a continuous

sidewalk.   According to the MUTCD, alternative routes that require

pedestrians     to   cross      a   roadway    are      discouraged,   but     (if

unavoidable)    such     routes     should   include     "advance   signing"   to

encourage        safe           travel         across        the       roadway.

Importantly, pedestrians should not be confronted with "midblock

worksites" that "will induce them to attempt skirting the worksite


     7 According to the U.S. Department of Transportation, the
MUTCD is a set of guidelines used by "road managers nationwide to
install and maintain traffic control devices on all public streets,
highways, bikeway, and private roads open to public travel." MANUAL
ON  UNIFORM  TRAFFIC  CONTROL  DEVICES  FOR   STREETS AND  HIGHWAYS,
https://mutcd.fhwa.dot.gov/index.htm.      Moreover,   the    MUTCD
compiles "national standards for all traffic control devices,
including road markings, highway signs, and traffic signals" and
is updated periodically by the Federal Highway Administration.
Id. The current MUTCD edition was published in 2009. Id.


                                      - 10 -
or making a midblock crossing."               To "minimize[e] the possibility

of   midblock        crossings,"    the    MUTCD    advises      that   TTC    devices,

including temporary traffic barriers or "longitudinal channelizing

devices," may be used.             But whenever feasible, "closing off the

worksite from pedestrian intrusion" is preferable to channelizing

foot       traffic   with   TTC    devices.        The   MUTCD    (as   a     whole)   is

incorporated by reference into CSA's MOT (recall that the MOT

governed the management of vehicular and pedestrian traffic in the

area where Lawes' accident occurred).8

               As to the data Aronberg had considered, his report

references Lawes' police accident report; photos taken by police

on the scene; daytime and nighttime aerial photos of the accident

site; observations from Aronberg's inspection of Riviere-Andino's

SUV and the accident site; unspecified construction "plans"; the

2003 and 2009 editions of the MUTCD; lighting records produced by

PREPA (purportedly showing that certain streetlights in the area

were under maintenance and not operational the night of the

accident); and a description of Lawes' activities before the

accident (provided by Lawes' counsel).



       8Specifically, General Note 4 of the MOT states:
"[p]rovisional signing and control of traffic shall be as provided
in the Manual on Uniform Traffic Control Devices, Part VI . . .
and with Specification 638 listed on Standard Specifications for
Road and Bridge Construction." We'll go over the relevance of the
MUTCD and Specification 638 to this case in more detail later.



                                          - 11 -
             After   providing       a    summary    of    the   relevant       facts,9

Aronberg's report ends with one long paragraph of analysis, which

discloses the following opinions:

             1.     The    "contractor"          (Q.B.)    created     a   dangerous

condition that contributed to Lawes' accident "by not providing a

safe sidewalk and/or positive guidance in the use of sidewalks."

             2.     Given the lack of "positive guidance," Lawes' path

on the night of his accident was reasonable.

             3.     The contractor did not take steps to ensure the

area streetlights were functioning that night.

             4.     The contractor should have closed the southern

sidewalk completely with a barricade starting at the intersection

(where the crosswalk was located).               Positive guidance "in the form

of a sign reading 'SIDEWALK CLOSED CROSS HERE'" could have been

attached to the barrier.

             5.     In view of his opinion about closing the southern

sidewalk     completely,      Aronberg      opined        that   the    southernmost

eastbound    lane    of    traffic       could    have    been   blocked    off       and

repurposed    as     a    continuous      temporary       sidewalk     (known    as    a


     9 Aronberg's factual summary was derived in part from the
police accident report. How do we know? It contains the police
report's erroneous cardinal directions (i.e., Aronberg's report
says south when it should say north and says east when it should
say west (and vice versa)). Aronberg later clarified that he was
aware of this issue (as of his visit to the site) but nevertheless
adopted the police accident report's directions for consistency.



                                         - 12 -
"pedestrian corridor").     Left open, the lane was too narrow for

vehicular traffic anyhow.10

          Crucially, these opinions were primarily directed at

Q.B., the contractor.       The report concludes by claiming the

contractor "and others" proximately caused Lawes' accident, but

does not mention anyone else by name.11 Aronberg, however, reserved

the right to modify his report in light of sworn testimony or other

evidence produced during discovery.

                     Aronberg's First Deposition

          Speaking    of   sworn    testimony,   eleven   months   after

publishing his report, Aronberg was deposed for the first time on

November 8, 2013.    Riviere-Andino, the Municipality of San Juan,




     10To be clear: Aronberg at no point suggested that the narrow
southernmost eastbound lane caused Riviere-Andino to hit Lawes,
which makes sense because Riviere-Andino was traveling in the
westbound lane when the accident occurred. Rather, as Aronberg
clarified in depositions and during his Daubert hearing, the lane's
narrowness was relevant because it could have been closed to
vehicular traffic and converted into a continuous sidewalk for
pedestrians (in accordance with MUTCD guidance and other industry
standards).    In Aronberg's expert opinion, providing such a
pedestrian corridor would have been safer than leaving the
partially-blocked sidewalk open, which he believed induced Lawes'
jaywalking.
     11At the time the report was published, CSA was not a direct
defendant in the litigation.


                                   - 13 -
and Q.B. sent their lawyers to the deposition, which lasted six

hours.

             Q.B. began with a line of questions about Aronberg's

background,    qualifications,         and    methodology.      When   given   the

opportunity to spell out his methodology, Aronberg explained that

he had applied the MUTCD to the conditions in place during Lawes'

accident to determine "the devices that should have been used and

the manner in which [Q.B.] should have implemented a traffic

control plan to account for pedestrian traffic."                Aronberg claimed

that his MUTCD-based methodology is generally accepted by traffic

engineers,     and    it   was    validated        by   the    Federal    Highway

Administration,       a    division          of   the   U.S.     Department     of

Transportation that publishes the MUTCD.                When asked whether his

methodology had been reviewed by others in the industry, Aronberg

mentioned     the    Institute    of    Transportation        Engineers   Journal

("ITE") as an example.      He insisted, however, that he did not need

to rely on the ITE or other publications to know that the MUTCD

applied.    Rather, experts in the field need only look to the MUTCD

itself, which Aronberg described as both the "law" and the "Bible"

for traffic engineers.           For Aronberg, the MUTCD serves as the

ultimate litmus test for evaluating traffic control plans for

pedestrian safety.

             After being quizzed about his methodology, Aronberg

listed the data he considered in forming his expert opinions, which


                                       - 14 -
was         mostly      consistent       with      his    preliminary       report.

Aronberg revealed for the first time, however, that he had measured

the width of Fernández Juncos' southernmost eastbound lane (from

the yellow lane divider to the beginning of the midblock barrier)

during       his     July   16,   2012    site    inspection.        Aronberg    also

volunteered information about new data he had considered since

publishing his report.            His new sources included video animations

of    what    he     thought   occurred    the    night   of   the   accident;   the

guidelines issued by the American Association of State Highway

Transportation Officials ("AASHTO")12 (which set standards for

highways, including the southernmost eastbound lane that Aronberg

believed was too narrow, and which are incorporated by reference

in the MUTCD); and CSA's MOT (which Aronberg did not have access

to in full when writing his preliminary report).                 He also reviewed

deposition testimony from Ruth Vargas (CSA's lead MOT designer)

and Edgardo Velez (Q.B.'s project manager).


       12
        As Aronberg explained during his first deposition: "AASHTO
is a nonprofit, nonpartisan association representing highway and
transportation departments in the 50 states, the District of
Columbia, and Puerto Rico . . . . Its primary goal is to foster
the development, operation, and maintenance of an integrated
national transportation system." According to the U.S. Department
of Transportation, AASHTO serves as "a liaison between State
departments of transportation and the Federal government," and
produces manuals and other guidance concerning (among other
things)    the  national    standards  for   roadway   design   and
installation.          U.S.    Department    of     Transportation,
https://safety.fhwa.dot.gov/roadway_dept/countermeasures/reduce_
crash_severity/aashto_guidancecfm.cfm (last visited June 17,
2020).


                                         - 15 -
            Defense    counsel's       microscope   eventually      turned   to

Aronberg's expert opinions. From under the lens, Aronberg unveiled

some new opinions, and added color to the opinions disclosed in

his report.        We'll tackle the "new" opinions first.              Aronberg

stated, for the first time, that there should have been "something"

in place preventing Lawes from crossing at midblock from the

northern sidewalk to the southern sidewalk.             In particular, orange

safety    fences    should   have    been   installed    along   the   northern

sidewalk to prevent pedestrians from jaywalking to the open part

of the southern sidewalk after the midblock barrier.                He pointed

out that this opinion was consistent with CSA's MOT.                The MOT, in

addition to providing the contractor with a blueprint of how the

construction-affected area should look, includes twelve "General

Notes."     Note 11, according to Aronberg, supported his orange

safety fence opinion.        It states:     "orange safety fences shall be

installed between the sidewalk and the working area for the safety

of the pedestrian flow."13          Aronberg's other new opinion was that

Q.B. should have monitored the area for dangerous conditions.

Had Q.B. been on proper daytime and nighttime monitoring duty, as

Aronberg   suggested,    it    would    have   detected    unsafe    pedestrian

conduct before Lawes' tragic accident.



     13The MUTCD also calls for the implementation of "pedestrian
channelizing"   devices,  where   needed,   to  "minimiz[e]   the
possibility of midblock crossings."


                                     - 16 -
            Aronberg also provided more substance to his previously

disclosed opinions.      First, he elaborated that "positive guidance"

is important because pedestrians are more likely to take a shorter,

more dangerous route to their destinations if left to their own

devices.      When asked to provide the basis for his conclusions

regarding pedestrian behavior in construction zones, Aronberg

explained that he (and other traffic engineers) received training

on "human factors" that helped predict "how people are going to

conduct    themselves,   whether    it   be    foot    traffic    or   vehicular

traffic."14    Aronberg also noted that the deposition testimony of

sailors who admitted to jaywalking to and from the southern

sidewalk supported his opinion that the area impacted by the Bahía

Urbana project lacked adequate positive guidance.                  Based on the

excerpts from Aronberg's deposition in the record, his deposers

did not ask him to elaborate further on what positive guidance

should have been in place.

            Second,   considering    new      data    he   had   received   since

submitting his report, Aronberg expounded on his opinion that the


     14At the time, Aronberg was not asked by any deposer whether
the MUTCD supported his opinion. However, the MUTCD specifically
"recognize[s] that pedestrians are reluctant to retrace their
steps to a prior intersection for a crossing or to add distance or
out-of-the-way travel to a destination." The MUTCD, therefore,
cautions   against   "midblock    worksites   that    will   induce
[pedestrians] to attempt skirting the worksite or making a midblock
crossing," and it advocates in favor of "appropriately" directing
pedestrians with guidance such as "advance signing."



                                   - 17 -
southern sidewalk should have been completely closed.                    When he was

drafting his preliminary report, he only had access to "some pages"

of the construction "plans."15               He eventually received the full

MOT, which was an exhibit to (CSA MOT designer) Ruth Vargas'

deposition.          With the help of the MOT and Vargas' deposition

testimony, Aronberg now understood that the MOT left the sidewalk

partially open to accommodate a planned bus stop that was being

relocated        from      an         area       closed       for      construction.

Aronberg, therefore, did not "fault" Vargas (and, by extension,

CSA)    for   designing        the    midblock    barrier     and   partially    open

sidewalk.       He continued to think, however, that closing the

sidewalk      completely        was     generally       the    safer    option    for

pedestrians.         And notwithstanding the bus stop, Aronberg still

believed      Q.B.    should    have    done     more    to   ensure   the   design's

implementation was safe for pedestrians.                  Aronberg explained, for

example, that Q.B. could have initiated a "request for information

process" to identify alternatives to the midblock barrier, and it

could have installed the safety fence (part of his new opinion)

along the northern sidewalk to prevent midblock jaywalking.

              Third,    Aronberg       described        the   pedestrian     corridor

opinion in greater detail.               Recall that the preliminary expert

report concluded (without elaboration) that Fernández Juncos'


       15
        It is not clear from the record what plans Aronberg had
access to when forming the opinions disclosed in his report.


                                         - 18 -
southernmost eastbound lane of traffic was too narrow and could

have been closed due to the midblock barrier that jutted into it.

When asked how he knew the lane was too narrow, Aronberg stated

that he measured the lane during his July 2012 inspection.          Based

on his measurements, the lane was less than 8 feet wide.               He

understood from AASHTO references that the lane ordinarily should

have been at least 10 feet wide.             Aronberg believed that a

construction contractor like Q.B. should have noticed the lane was

too narrow when it constructed the midblock barrier.          Since the

lane was unsafe, Q.B. should have "approach[e]d the [Puerto Rico]

highway authority and the designers to determine what should be

done."   A reasonable next step, as Aronberg tells it, was to close

the unsafe lane and convert it into a pedestrian corridor.

            Finally, Aronberg expanded his report's one-sentence

opinion that Q.B. had to ensure the streetlights were working in

the area.    When pressed about Q.B.'s authority over streetlights

(or lack thereof), he claimed the contractor was responsible for

ensuring    adequate   lighting   in   the   construction   "work   area"

regardless of the maintenance work that PREPA was conducting the

night of Lawes' accident.    For support, he pointed to his years of

experience in the industry, an unspecified part of the MOT, and

Vargas' deposition, in which she explained that Q.B.'s area of

responsibility included the northern sidewalk.       That's it for the

first deposition.


                                  - 19 -
                    Aronberg's Second Deposition

            On August 21, 2014, Aronberg was deposed a second time.

There was one major development in the litigation at the time:

Lawes amended his complaint to include allegations against CSA.16

CSA, the new defendant on the block, sent counsel to depose Lawes'

expert.17

            As was the case for his first deposition, Q.B. conducted

the first round of questioning. Out of the gate, Aronberg revealed

he had considered new data, namely:     the July 2014 expert report

published by Murray Yates on behalf of CSA;18 the deposition of

Riviere-Andino's accident reconstruction expert, Steven Schorr

(who testified that the construction-affected area would have been




     16In his successful motion for leave to file a third amended
complaint naming CSA as a defendant, Lawes stated that "after
further discussions with [Aronberg]" he had concluded that CSA's
"poor design of the construction contributed to the accident in
this case."
     17 The other parties present were:       Q.B.; Bonilla (the
engineering inspection firm retained in connection with the
Project); PREPA (the municipal agency responsible for maintaining
the light poles near Piers 7 and 8); the Municipality of San Juan;
and two of their insurance carriers.
     18 Yates' report states that he has expertise in "construction
engineering," as well as the design, inspection, and evaluation of
temporary traffic control plans. The report concludes that CSA
had a duty to reasonably design the MOT and it fulfilled that duty;
Q.B. was responsible for implementing CSA's design; and Bonilla
was obligated to monitor the site.       Stay tuned because we'll
discuss Yates' report and the report published by Q.B.'s expert
after we wrap up Aronberg's second deposition.



                               - 20 -
safer if pedestrians were not required to cross any lanes of

traffic); the report and deposition of Riviere-Andino's other

accident      reconstruction       expert,     Iván   Baigés   Valentín    (who

testified the signage along the southern sidewalk was inadequate);

the depositions of Paul Levergne Arostegui and Francisco Bechara

Rivera;19 the deposition of Grandvill Lawes; a second deposition

of   Riviere-Andino;        an    American    National    Standards   Institute

document pertaining to lighting and signage; Q.B.'s requests for

information ("RFIs") from CSA and others during the implementation

of the MOT; and minutes from meetings attended by Q.B., CSA, and

other      parties   to     the    Bahía     Urbana   construction    contract.

In addition, Aronberg visited the scene of the accident again,

this time at night.20

             Aronberg openly acknowledged he had developed some new

opinions, modified some earlier opinions, and put a finer point

(i.e.,     more   detail)    on    other     previously   expressed   opinions.

Perhaps the most important new opinion for our purposes:               Aronberg

now opined that CSA's "poor" design contributed to Lawes' accident.


      19
       These individuals do not come up elsewhere in the portion
of the record pertinent to our review.
      20Three months before his August 2014 deposition, Aronberg
inspected the scene at night. He was not asked to provide the
exact time of his visit.     But he did explain that there were
"lights in the area" that he believed were not there before.
Recall that Aronberg opined in his preliminary report and prior
deposition that the streetlights along the northern sidewalk were
not operational at the time of the accident.


                                      - 21 -
In other words, Aronberg offered the expert opinion that the MOT

was negligently designed.       For support, he identified two major

flaws.

           First, according to Aronberg, the MOT failed to properly

instruct Q.B. on implementing "positive guidance" for pedestrians.

Although   the   MOT    included   "notes   that   a   contractor    could

implement," those instructions were not detailed enough.                  In

Aronberg's opinion the "best practice" was to "provide the positive

guidance [and] not leave things up to the contractor who might not

have the expertise of the actual designer."        So the design should

have specifically instructed Q.B. to implement         a "fence" or other

"physical barrier," as well as signs that would assist pedestrians

in   following   a   safe,   MOT-approved   footpath   to   and   from   the

waterfront.21    Although he did not cite any MUTCD provision that

applied, he explained that his opinion was consistent with the

MUTCD's purpose.22

           Second, CSA was negligent by including the midblock

barrier in its design.         In contrast to his report and prior


      21He explained, for example, that the MOT lacked "signs
guiding pedestrians [on the northern sidewalk] to go to the
crosswalk at the traffic signal before crossing the roadway."
These signs might say: "Crosswalk Further Ahead," "Detour," or
"Pedestrian Detour."
      22
       At another point in his deposition, Aronberg elaborated on
his positive guidance opinion at a more general level.         He
emphasized, as he had in his first deposition, that positive
guidance is necessary because pedestrians (when left to their own



                                   - 22 -
deposition testimony, both of which blamed Q.B. for errors in

implementing the MOT, Aronberg concluded that CSA's negligence

also extended to the midblock barrier.                 Why?   Following his last

deposition, Aronberg had received and reviewed Q.B.'s RFIs and

responses from the Project.           One such RFI response from the Puerto

Rico Metropolitan Bus Authority indicated that the planned bus

stop was not going to be relocated to the southern sidewalk after

all.        And   Project   meeting    minutes,    which      Aronberg   had   also

reviewed, indicated that both Q.B. and CSA were present for a

discussion of the decision not to relocate the bus stop to the

southern sidewalk.          Having solved the bus stop mystery, Aronberg

now concluded: (1) there was no justification for the midblock

barrier,      which   Aronberg    viewed    as    an    unsafe   condition     that

contradicted MUTCD guidance;23 and (2) assuming CSA's MOT designers

knew the bus stop was not going to be relocated to the southern

sidewalk,24 CSA had the duty to act, including redesigning the MOT

with a closed sidewalk.


devices) are more likely to take a dangerous, shorter path through
a construction zone.    To bolster his expertise on the subject
matter, Aronberg testified about his extensive training on "and
observation of" pedestrian behavior in construction-affected
areas.
       23
        In Aronberg's professional opinion, the MUTCD allows
sidewalks to be closed at mid-block if and only if "there's
something that the sidewalk serves."
       24
       When pushed by CSA's counsel, Aronberg conceded that he did
not know whether the CSA representatives at the meeting were



                                      - 23 -
          Adding   to   his   new    theories   about   CSA's   negligence,

Aronberg claimed that when CSA was designing the MOT and first

learned a bus stop was going to be situated in the construction-

affected southern sidewalk, it "should have at least requested .

. . permission" from the Puerto Rico Metropolitan Bus Authority to

put the bus stop somewhere else.         In Aronberg's opinion, CSA had

a "professional duty . . . to design something safely," including

by taking the affirmative step of contacting municipal authorities

to request a different location for the bus stop so that Q.B. could

close the sidewalk completely.          Aronberg was asked whether his

opinion conflicted with the MUTCD, which includes one model traffic

control design in which the sidewalk is left partially open in a

construction-affected area.         He countered that, pursuant to the

MUTCD, such a design is only appropriate "[i]f there was a reason

that the sidewalk has to be left open."25 Here, given CSA's options




involved with the MOT design or whether they focused on other parts
of the Project. So he could not say for sure whether the changed
bus stop plans were passed on to the design team. While he believed
CSA's MOT designers had a duty to know "what was happening with
their design," he elected to "only fault them if they knew" the
bus stop was not going to be relocated.
     25The MUTCD states, for example: "Whenever it is feasible,
closing the worksite from pedestrian intrusion may be preferable
to channelizing pedestrian traffic along the site with TTC
devices." MUTCD Section 6D.12; see MUTCD Section 6D.09 (discussing
strategy for minimizing midblock crossings).



                                    - 24 -
and the conditions in place, the sidewalk should have been closed

completely.

               Later on, Aronberg was asked to clarify whether his new

opinions regarding CSA's negligent design extended to the narrow

eastbound lane of traffic, which Aronberg thought could be closed

and converted into a pedestrian corridor.         In Aronberg's opinion,

the design (albeit imperfectly drawn)26 instructed Q.B. to keep the

lane width as is.        And, even if Q.B. mistakenly thought the design

required the encroachment, it should have followed up to confirm

before implementation.           Aronberg thus blamed Q.B.'s negligent

implementation (not CSA's negligent design) for the narrow lane.

To avoid confusion down the line, we'll stress here that the lane's

width was only relevant insofar as it indicated that the lane could

be closed and converted to a better use:              providing pedestrians

like Lawes with a safe, continuous sidewalk to the south of

Fernández Juncos.        In Aronberg's book, Q.B. was to blame for not

making the connection between the narrow lane (which it should

have        identified   while   implementing   the     barrier)   and   the

opportunity to implement a safer alternative for pedestrians.            And

since Q.B. was the Project's lead contractor, Aronberg concluded



       26
        Aronberg acknowledged that there were issues with CSA's
drawn-to-scale depiction of the construction-affected area.    He
noted, for example, that it did not provide the actual dimensions
of Fernández Juncos' lanes, and there was a lane down the block
from Lawes' accident that the MOT got wrong.


                                    - 25 -
that it should have taken affirmative steps to close the lane from

traffic even if it meant contacting the Puerto Rico Highway and

Transportation Authority to request permission to do so.

             Apart    from   the   above-mentioned   new   and   amended

opinions,27 Aronberg mostly doubled down on those he had previously

expressed.     For instance, when defense counsel pointed out that

the regulation of streetlights in Puerto Rico falls within the

jurisdiction of the federal government, Aronberg responded that

Q.B., as the construction contractor, still had lighting-related

responsibilities.       Aronberg said that he did not need to review

the law to support this opinion.       Rather, the MOT itself dictates

that contractors are responsible for adequate illumination of

their area.28        At a different point, Aronberg touched upon his




     27While we don't need to spill much ink on this because of
its tangential relevance, Aronberg also opined about the
negligence of Bonilla, the project's safety inspector and third-
party defendant in the district court case. Although he believed
Bonilla missed the mark in performing its inspection duties, he
still claimed that Q.B. should have routinely monitored the area.
     28The excerpts in the record do not indicate whether Aronberg
pointed out the applicable section of the MOT to support this
opinion. Relevant here though, MOT General Note 10 instructs Q.B.
to read Specification 638 of the Standard Specifications for Road
and Bridge Construction issued by the Puerto Rico Highway and
Transportation   Authority.      Specification   638  "puts   [the
contractor] [o]n notice that he has the primary responsibility for
providing the necessary traffic control devices and taking other
appropriate measures for the protection of the public and his
personnel."



                                   - 26 -
opinion that there should have been an orange safety fence29 along

the northern sidewalk to "channelize pedestrians" and discourage

midblock crossings.

          Aronberg    was    forthcoming    about   the   fact   that   he

considered the August 2014 deposition to be an opportunity to

supplement his opinions.      Asked by Q.B.'s counsel if he intended

to provide a supplemental written report, Aronberg replied:             "I

would only amend it if I'm asked to amend it.         To me, I've given

a report and I've supplemented the report in depositions, which is

sworn testimony for everybody to read.          I don't know that I'm

required to put it in any further written form beyond that."

Later, CSA's lawyer asked Aronberg:        "I just want to make sure to

understand that I guess your final report would be your preliminary

report and your testimony in the depositions.             Is that a fair

assessment?"   Aronberg's "yes" in response was unequivocal.

                            The Other Experts

          Defendants also put up experts, who agreed with some of

Aronberg's opinions and attempted to cast doubt on others.          We'll

talk about a few of them.       First up, CSA's traffic engineering



     29During Aronberg's second deposition, it became clear that
the orange safety fence opinion was the offspring of his more
general opinion that the construction-affected area lacked
positive guidance for pedestrians. To avoid confusion, however,
we will continue to discuss the orange safety fence opinion
separately because of what happened down the road.



                                 - 27 -
expert, Yates, concluded in his July 2014 report that CSA's MOT

design was reasonable, and that Q.B. had primary responsibility

for   the    design's     implementation,    according    to   the    standard

specifications30 (and other such "contract documents") referenced

in the MOT's General Notes.          During Yates' first deposition on

August 23, 2014 (two days after Aronberg's second deposition), he

explained that the standard specifications and the MUTCD required

Q.B. to monitor the construction area to ensure pedestrians were

safely navigating it -- which aligns with Aronberg's monitoring

opinion.      Notwithstanding Q.B.'s monitoring obligations, Yates

opined that "all the parties" (including CSA, Q.B., and the

project's safety inspector, Bonilla) "should be looking at the job

and verifying whether the conditions were attributing to [unsafe

pedestrian patterns] in some way."            When he was asked about the

utility of Aronberg's "orange safety mesh" fence, however, Yates

explained that such barriers would not have made a difference here.

             On August 30, 2014, days after Aronberg's second and

Yates'     first     deposition,   Q.B.'s    traffic    engineering   expert,

Hanscom, released a report.         Hanscom, per his report, previously

served on the National Committee for Uniform Control Devices, which

helps the Federal Highway Administration draft the MUTCD.              Hanscom

concluded     that    Lawes'   "crossing    behavior"    (i.e.,   jaywalking)



      30   Relevant here, Specification 638.


                                    - 28 -
caused his accident, and neither additional signage nor adequate

street illumination would have prevented it.          He also claimed that

Aronberg had failed to articulate why the width of the southernmost

eastbound lane was relevant to Lawes' accident.             Exactly three

months later, on November 20, 2014, Hanscom filed a supplemental

report "in response to depositions" he had reviewed after filing

his initial report.         In particular, Hanscom finally got the

opportunity to review Aronberg's and Yates' deposition testimony.

As is relevant here, Hanscom's supplemental report claimed that:

(1) Lawes' negligence caused the accident; (2) CSA and Q.B. were

not   responsible     for   providing     "positive    guidance"   on    the

unobstructed    northern    sidewalk;     (3)   Bonilla   (not   Q.B.)   was

responsible for "inspection activity," which included observing

"pedestrian flow activity"; (4) the midblock barrier did not force

pedestrians    into   a   "pattern   of   dangerous   walking    behavior";

rather, "abundant evidence" indicated that they were trying to

avoid a dangerous area along the northern sidewalk (i.e., the hot

corner); (5) signage location, the midblock barrier, and the narrow

eastbound lane did not contribute to Lawes' accident and, even if

they did, "the placement of those items is an issue of [CSA's MOT]

design"; (6) because Q.B. was responsible for illuminating the

work area only when construction was underway, it had no lighting

obligations at night, when Lawes' accident occurred; and (7) Q.B.

was not authorized to modify the MOT without CSA's permission.


                                 - 29 -
Hanscom     doubled    down     on   most   of   these   opinions     during   his

deposition on January 29, 2015, which Lawes and CSA attended.31

However, when confronted with MUTCD Section 6B, which requires

that construction-affected areas undergo routine day and night

inspections, he acknowledged that the MUTCD was ambiguous as to

whether Q.B. (the contractor) or Bonilla (the inspector) was

required to conduct such monitoring of the area while the MOT was

in effect.

             Separately,       Riviere-Andino's       accident   reconstruction

expert,32    Ivan     Baigés    Valentín,    inspected    the    accident   site,

measured the width of the lanes, conducted perception visibility

analysis    (including     an    evaluation      of   lighting   in   the   area),

calculated Riviere-Andino's speed reaction time, and prepared an

accident reconstruction diagram.             The main thrust of his expert

report (which is described but not included in the record) and

June 2014 deposition testimony was that Riviere-Andino was not



     31 Q.B. also retained a professional engineer, Dennis
González. The bulk of González's expert report from August 29,
2014 is dedicated to distinguishing Q.B.'s project-related
responsibilities from Bonilla's; he explained that Bonilla (not
Q.B.) was in charge of monitoring the construction-affected area
after the MOT's implementation. González also claimed that PREPA
(not Q.B.) was responsible for insufficient lighting (if any).
     32Riviere-Andino's other expert, Steven Schorr, published an
expert report on May 20, 2013 and was deposed on November 11, 2013.
The record provides minimal insight into his opinions and
methodology.



                                      - 30 -
responsible for Lawes' "actions" or the "hazardous conditions" in

the area.       But, according to Baigés Valentín, the lack of street

lighting along the northern sidewalk impacted Lawes' visibility

(to Riviere-Andino), and inadequate signage was also an issue.33

                        The Trial and the Daubert Hearing

               On March 7, 2016, following the close of several years

of   discovery,       a    jointly     filed     188-page    pretrial         order,     the

"longest" pretrial conference in the district court's twenty-three

years on the bench, and prodigious motion practice, the trial on

Lawes'      claims   began.       It    lasted     twenty-eight            days,   and   was

litigated by twelve, and sometimes thirteen, lawyers on any given

day.

               On the fifteenth day of the trial, April 19, 2016, the

court       commenced     what   turned    out     to   be   a    twelve-day       Daubert

hearing,34 with the purpose of determining whether or not Lawes'

expert       could   testify     before    the     jury.         It   is    important     to

underscore that, although defendants had made some efforts to

restrict Aronberg's testimony in their motions in limine, no party



       33
       With respect to signage, Baigés Valentín clarified during
his deposition that he believed more signage was needed along the
southern sidewalk.
       34
        The Daubert hearing concluded on May 16, 2016, after
thirteen days on the court's docket. But since Aronberg did not
appear in court and was not questioned on April 22 (when the court
heard the parties on Lawes' motion to disqualify the judge), we do
not count that day.


                                          - 31 -
had formally moved to exclude his testimony in its entirety.                In

fact, the court opened the hearing by pointing out that, even

though there was no Daubert challenge on the docket, "plaintiff

has made arguments to the Court that have obligated the Court to

hold a Daubert hearing."

           That same day, Q.B. (via its insurer) complained for the

first time that Aronberg was not a reliable witness under Federal

Rule of Evidence 702, and that Lawes had insufficiently disclosed

Aronberg's expert opinions prior to trial in violation of Federal

Rule of Civil Procedure 26(a)(2)'s disclosure requirements and

Rule   26(e)(2)'s     supplementation      requirements.          From   Q.B.'s

perspective, when Aronberg started to explain its responsibility

to place an orange mesh fence between the northern sidewalk and

the combined roadways' many lanes, the expert was expressing an

opinion    that     was    unreasonably     speculative     and     previously

undisclosed.      The very next day, Q.B. filed a motion in limine to

sanction Lawes for his Rule 26 disclosure violations pursuant to

Rule   37(c)(1),     and   requested      the   court   exclude     Aronberg's

testimony under Rule 702.          Lawes argued in his opposition that

defendants had not demonstrated surprise and prejudice (factors

favoring   preclusion      under   Rule   37(c)(1))     since   Aronberg   had

expressed the orange safety fence opinion during both of his

depositions, defendants' experts had rebutted the opinion in their




                                   - 32 -
reports and depositions,35 and the opinion was disclosed in the

parties'   jointly-filed   pretrial    order.      Moreover,    the   orange

safety fence opinion was supported by sufficient facts and data to

withstand Q.B.'s first official Daubert challenge.             Nonetheless,

the   district   court   issued   an   order    explaining   that,    to   be

admissible at trial, Aronberg's opinions must have been included

in his preliminary expert report or a written supplemental report.

Since the orange safety fence opinion was not disclosed in a

report, the court found that Lawes had violated Rule 26's expert

disclosure and supplementation requirements.        Moreover, because of

Lawes' "ambush litigation tactics," his misconduct required the

exclusion of all orange safety fence-related testimony.           The court

reserved for another day its opinions on Aronberg's reliability.

           The hearing's civility deteriorated from there.                 The

attorneys pushed and shoved one another and, alarmingly, one

defense counsel purportedly suggested he and plaintiff's counsel

take their courtroom drama "downstairs."         At times, Aronberg, the

hearing's only witness, could not get a word in edgewise.              Near

the end of this unusually eventful Daubert hearing, several motions

were filed to preclude Aronberg's testimony as a sanction for



      35
       CSA's expert opined during his deposition that placing such
a fence in the median between the northern sidewalk and Fernández
Juncos would not have prevented Lawes' accident since "mesh fences
are about 30 inches in height" so "all an individual has to do is
step over it."


                                  - 33 -
Lawes' discovery violations (motions that are typically filed and

resolved before trial), and/or to exclude Aronberg from trial under

Federal Rule of Evidence 702.    These motions were granted on June

23, 2016, and Lawes was ordered by the court "to show cause"

whether or not he would proceed even though "the Court . . .

excluded the only witness that could establish causation between

Defendants' acts and Plaintiff's accident."     The district court

nevertheless allowed Lawes to soldier on, and he finally rested

his case on July 6, 2016.       Defendants moved for judgment as a

matter of law under Federal Rule of Civil Procedure 50(a), which

the district court allowed, ending Lawes' case.

           On October 10, 2016, Lawes timely appealed his expert's

exclusion from the litigation and the entry of judgment against

him.   That brings us to the present.

                             OUR TAKE

           We'll kick things off by explaining the role of expert

testimony in Lawes' case.   Under Puerto Rico law, which supplies

the substantive law in this diversity case, Lawes "had to show

'damage . . . through fault or negligence.'"    Aponte-Bermúdez v.

Colon, 944 F.3d 963, 963-64 (1st Cir. 2019) (citing Rodríguez-

Tirado v. Speedy Bail Bonds, 891 F.3d 38, 41 (1st Cir. 2018), and

quoting P.R. Laws Ann. tit. 31, § 5141).    Lawes argued that CSA,

the sole remaining defendant, negligently designed the MOT and

caused his injuries.    Lawes therefore needed to establish that


                                - 34 -
"[CSA] owed [him] a duty," "that the duty was breached, that

damages resulted, and that those damages were caused by the

breach[.]"      Calderón-Ortega v. United States, 753 F.3d 250, 252

(1st Cir. 2014). In negligent design cases, like this one, experts

are needed to educate the jury on the industry-specific standard

of care that applied.            See Aponte-Bermúdez, 944 F.3d at 964

(explaining that in negligent design cases "under Puerto Rico law,

[plaintiffs]     would      ordinarily     have   to   prove    the   applicable

standard   of    care      through   expert    witnesses"      (citing     Vázquez-

Filippetti v. Banco Popular de Puerto Rico, 504 F.3d 43, 51-52

(1st Cir. 2007))).            We have explained that determining what

constitutes a reasonably safe design is "ordinarily 'beyond the

experience or knowledge of an average lay person.'"                 Id. (quoting

Vázquez-Filippetti, 504 F.3d at 52).              What is "'ordinarily' true

is not invariably true," so there may be exceptional cases in which

the "negligence in design [is] blatant enough not to require expert

testimony[.]"        Id.     In this case, after excluding Lawes' only

expert, the district court concluded that expert testimony was

essential to establishing the applicable standard of care, which

Lawes had to do as a matter of law.            Lawes v. Q.B. Constr., No. CV

12-1473 (DRD), 2016 WL 4660915, at *4 (D.P.R. Sept. 7, 2016)

(stating that "a lay jury certainly may not rely on personal

experience and knowledge to establish the duty of care owed to

pedestrians     by   designers       of   temporary    management     of   traffic


                                      - 35 -
plans").      Then, the district court dismissed Lawes' case.            For

reasons we'll explain later, we do not reach the district court's

entry of judgment on the merits for defendants.             Instead, we'll

focus our energy on the rulings that resulted in Aronberg's

exclusion from trial in the first instance, keeping in mind the

district   court's     musings   regarding    the   importance   of   expert

testimony to Lawes' case.

                           Standard of Review

              We review both of the rulings resulting in Aronberg's

exclusion from trial for an abuse of discretion.         Esposito v. Home

Depot U.S.A., Inc., 590 F.3d 72, 78 (1st Cir. 2009) (applying abuse

of discretion to expert's preclusion under Rules 26 and 37(c)(1));

Ruiz-Troche v. Pepsi Cola of P.R. Bottling Co., 161 F.3d 77, 83

(1st   Cir.    1998)   (reviewing   expert's    exclusion   after     Daubert

hearing for abuse of discretion).            Abuse of discretion "occurs

when a material factor deserving significant weight is ignored,

when an improper factor is relied upon, or when all proper and no

improper factors are assessed, but the court makes a serious

mistake in weighing them."       Fashion House, Inc. v. K Mart Corp.,

892 F.2d 1076, 1081 (1st Cir. 1989) (quoting Ind. Oil & Chem.

Workers of Quincy, Inc. v. Procter & Gamble Mfg. Co., 864 F.2d

927, 929 (1st Cir. 1988)).          "This standard is not monolithic:

within it, embedded findings of fact are reviewed for clear error,

questions of law are reviewed de novo, and judgment calls are


                                    - 36 -
subjected to classic abuse-of-discretion review."   Bricklayers and

Trowel Trades Int'l Pension Fund v. Credit Suisse Sec. (USA) LLC,

752 F.3d 82, 91 (1st Cir. 2014) (quoting Ungar v. Palestine

Liberation Org., 599 F.3d 79, 83 (1st Cir. 2010)).    On abuse-of-

discretion review, we will reverse a trial court's decision if we

determine the judge committed "a material error of law" or "a

meaningful error in judgment."   United States v. Jordan, 813 F.3d

442, 445 (1st Cir. 2016) (citing Ruiz-Troche, 161 F.3d at 83).

          We review de novo the district court's judgment as a

matter of law under Rule 50. Blomquist v. Horned Dorset Primavera,

Inc., 925 F.3d 541, 546 (1st Cir. 2019) (citing Thomas & Betts

Corp. v. New Albertson's, Inc., 915 F.3d 36, 60 (1st Cir. 2019));

see Fed. R. Civ. P. 50.

          We acknowledge the district court's valiant effort to

effectively and fairly administer the discovery process in this

case.   However, our close review of the record persuades us that

the district court abused its discretion in refusing to allow

Aronberg to testify.   To be sure, Lawes' pretrial disclosures were

far from ideal.   Nevertheless, in reaching the decision to exclude

Lawes' sole expert, the district court undervalued or overlooked

significant factors and made serious missteps in balancing the

import of other factors relevant to its analysis.     We therefore

reverse Aronberg's exclusion and vacate the entry of judgment as

a matter of law (without ruling on the merits of that decision).


                               - 37 -
Although CSA is the sole remaining defendant on appeal, our review

is informed by the district court's assessment of Aronberg's expert

opinions and testimony as a whole.         Thus, when necessary, we will

discuss district court analysis that concerns former defendants

and third parties.   We'll begin our take with the discovery rules,

and we'll end with Daubert and Federal Rule of Evidence 702.

               Rule 26:     Expert Discovery Disclosure

           Rule 26 "is an integral part of the machinery devised to

facilitate the management of pretrial discovery."         Downey v. Bob's

Disc. Furniture Holdings, Inc., 633 F.3d 1, 5 (1st Cir. 2011)

(quoting Gómez v. Rivera Rodríguez, 344 F.3d 103, 112 (1st Cir.

2003)).   "Recognizing the importance of expert testimony in modern

trial   practice,   [Rule   26]   provide[s]    for   extensive   pretrial

disclosure of expert testimony."       Thibeault v. Square D Co., 960

F.2d 239, 244 (1st Cir. 1992).       Plaintiffs and defendants alike

must identify their expert witnesses and produce their experts'

reports by court-approved deadlines.        Fed. R. Civ. P. 26(a)(2)(A)-

(B).    An expert's report must include a "complete statement" of

all the expert's opinions and the "basis and reasons" for them;

the facts and data the expert considered; any exhibits the expert

intends to rely on; a list of cases from the last four years in

which the witness testified as an expert at trial or by deposition;

and the compensation the expert will receive in exchange for his

testimony in the case.       Fed. R. Civ. P. 26(a)(2)(B).         But the


                                  - 38 -
expert's work doesn't end there. Rule 26(e) then instructs parties

that expert disclosures "must be kept current."       Macaulay v. Anas,

321 F.3d 45, 50 (1st Cir. 2003) (citing Fed. R. Civ. P. 26(e)(1)).

During litigation, usually before trial, experts must supplement

their reports at the court's request or when a party learns that

its "disclosure or response is incomplete or incorrect" and "the

additional or corrective information has not otherwise been made

known to the other parties during the discovery process or in

writing."    Fed. R. Civ. P. 26(e)(1).         The duty to supplement

extends "to information included in the [expert's] report and to

information given during the expert's deposition."          Fed. R. Civ.

P. 26(e)(2).

            Complete   and   timely   disclosures   and   supplementation

ensure an even playing field, preventing any party from gaining an

"unfair tactical advantage" at trial.        Lohnes v. Level 3 Comm.,

Inc., 272 F.3d 49, 60 (1st Cir. 2001); see Licciardi v. TIG Ins.

Grp., 140 F.3d 357, 363 (1st Cir. 1998) (explaining that adherence

to Rule 26 averts "the heavy burden placed on a cross-examiner

confronted by an opponent's expert whose testimony had just been

revealed for the first time in open court" (quoting Johnson v.

H.K. Webster, Inc., 775 F.2d 1,7 (1st Cir. 1985)). In the interest

of fairness, district courts may sanction litigants who disregard

these obligations.     Pursuant to Rule 37(c)(1), incomplete or late

disclosures may result in (among other possible sanctions) the


                                 - 39 -
preclusion of the "relevant expert information . . . 'at a hearing,

or at a trial, unless the failure was substantially justified or

is harmless.'"        Esposito, 590 F.3d at 77 (emphasis added) (quoting

Fed. R. Civ. P. 37(c)(1)).          Preclusion is not strictly required,

however.       "When noncompliance occurs, the ordering court should

consider the totality of events and then choose from the broad

universe of available sanctions in an effort to fit the punishment

to the severity and circumstances of the violation."                      Young v.

Gordon, 330 F.3d 76, 81 (1st Cir. 2003) (emphasis added) (citing

Tower Ventures, Inc. v. City of Westfield, 296 F.3d 43, 46 (1st

Cir. 2002)).          And where preclusion "carrie[s] the force of a

dismissal,"      as   the   district      court    implied   it   did    here,   the

justification for this sanction must be "more robust."                   Esposito,

590 F.3d at 79 (citing Young, 330 F.3d at 81 (explaining that

"dismissal ordinarily should be employed as a sanction only when

a     plaintiff's     misconduct    is     extreme"       (citation     omitted)));

see    Tower    Ventures,    296   F.3d    at     45-47   (concluding     that   the

plaintiff's "serial violations" of the district court's scheduling

orders constituted extreme misconduct that warranted dismissal).

"[D]ismissal should not be viewed either as a sanction of first

resort or as an automatic penalty for every failure to abide by a

court order."         Young, 330 F.3d at 81.          Because dismissal "runs

counter to our 'strong policy favoring the disposition of cases on

the merits,'" this severe sanction "should be employed only after


                                       - 40 -
the   district   court   has   determined   'that   none    of   the   lesser

sanctions available to it would truly be appropriate.'"                Enlace

Mercantil Internacional, Inc. v. Senior Indus., Inc., 848 F.2d

315, 317 (1st Cir. 1988) (quoting Zavala Santiago v. Gonzalez

Rivera, 553 F.2d 710, 712 (1st Cir. 1977)).            Ultimately, "the

choice of an appropriate sanction must be handled on a case-by-

case basis."     Young, 330 F.3d at 81 (citing Tower Ventures, 296

F.3d at 46).

            Moving from the general to the specific, the trial court

found that Aronberg's preliminary report violated Rule 26(a)(2)

because it did not include all his opinions or the basis and

reasons for them; Aronberg didn't say how much he was getting paid

to testify; and the photographs Aronberg took of the accident site

were not attached to the report (though they were mentioned and

disclosed later in the litigation).         Adding to these violations,

Aronberg did not submit a written supplemental expert report after

his   two   depositions,   which   the   district   court    considered     a

violation of Rule 26(e).       The district court then considered the

severity of Lawes' misconduct, and imposed one of the harshest

sanctions available, precluding the expert's testimony despite

Lawes' tremendous need for it.

            Lawes does not argue on appeal that Aronberg's expert

disclosures satisfied Rule 26, so we won't spill any ink on the

question. As to Lawes' quarrel with the district court's sanction,


                                   - 41 -
the question "is not whether we would have imposed the same

sanction.     Rather, the question is whether the district court's

action was so wide of the mark as to constitute an abuse of

discretion."     Macaulay, 321 F.3d at 51.        Thus, "appellate panels

traditionally give district courts considerable leeway in the

exercise of the latter's admitted authority to punish noncompliant

litigants."    Young, 330 F.3d at 81.     In undertaking our review, we

look to "the history of the litigation, the proponent's need for

the challenged evidence, the justification (if any) for the late

disclosure, and the opponent's ability to overcome its adverse

effects."   Macaulay, 321 F.3d at 51 (citations omitted).         Although

analysis of each factor is relevant, the "focus of a preclusion

inquiry is mainly upon surprise and prejudice" to defendants.

Thibeault, 960 F.2d at 246-47 (affirming preclusion of expert

testimony where plaintiff's "eleventh-hour" change in theory days

before   trial   would   have   forced    the    defendant   to   rush   its

preparations).     Since surprise and prejudice serve as "important

integers" in our deferential review, it makes sense to start there.

Macaulay, 321 F.3d at 51.

            As the district court tells it, CSA was "caught off-

guard" (read:    surprised) by Aronberg's Daubert testimony that the

MOT was "sloppy." For support, the district court cited Aronberg's

Daubert testimony from May 10, 2016.            At that point in the day,

CSA was following up with Aronberg about his reactions to some


                                 - 42 -
documents Q.B. had cross-examined him on earlier.            For context,

the day before, Q.B. gave Aronberg a copy of the MOT, then showed

him a blueprint of the area where Lawes' accident occurred in its

original   (pre-construction      and   pre-MOT)   state.    Next,   Q.B.'s

counsel conducted an experiment while Aronberg was sitting on the

stand:     he placed the pre-construction blueprint over the MOT

design, and asked Aronberg his thoughts on the superimposition.

Q.B.   queried    whether   the   MOT's   drawn-to-scale    specifications

instructed Q.B. to place the midblock barrier partially in the

southernmost eastbound lane.       Aronberg did not agree that this was

the MOT's intent.      Rather, as he told Q.B., the "drawing" in the

MOT was "sloppy."      When CSA brought this testimony up during its

cross-examination of Aronberg on May 10, 2016, the expert confirmed

his belief that the MOT was "sloppy."

            The trial court also found surprising Aronberg's Daubert

testimony about CSA's responsibility to ensure the sidewalk was

closed at the intersection in its design.          To the district court's

dismay, Aronberg testified on May 9, 2016 that CSA was obligated

to contact the Municipal Bus Authority about the phantom bus stop

since it was the only reason CSA could not close the sidewalk

completely.      Later, when defense counsel brought up Vargas' trial

testimony from several weeks before, during which Vargas testified

that she didn't know the bus stop plans had changed, Aronberg

explained that someone from CSA (if not Vargas) should have been


                                   - 43 -
in communication with the Bus Authority to get the most up-to-date

information about the MOT design and implementation process. Since

this opinion was not contained in Aronberg's report, the district

court concluded that CSA must not have known about it before.        For

example, after CSA finally wrapped up its bus stop questions during

the Daubert hearing on May 10, 2016, the district court described

Aronberg's design defect opinion as "late" and asked CSA whether

it   agreed,   prompting   vigorous   verbal   opposition   from   Lawes'

counsel and a court-initiated 10-minute recess.             In view of

Aronberg's Daubert testimony as to CSA'S design, the trial court

concluded that Aronberg was a moving target, and so it found that

CSA was prejudiced as a result.       As the district court explained,

the "prejudice . . . lies in the basis Aronberg provided for

imposing liability on CSA at the Daubert hearing[:] engineering

common sense."    We favorably assume the district court meant that

CSA's ability to test and confront Aronberg's conclusions at trial

was thwarted by the expert's perceived shiftiness and Lawes'

discovery violations.

           Here's the rub: Lawes' pretrial disclosures and relevant

excerpts from Aronberg's depositions (which Lawes attached to and

quoted in his motions opposing sanctions) gave CSA more than

sufficient notice of Aronberg's negligent-design-related opinions.

Based on this record, CSA was neither surprised nor prejudiced by

Aronberg's Daubert testimony.     Nearly two years before trial, on


                                - 44 -
June 20, 2014, Lawes requested leave to file a third amended

complaint in which he explained that, "after further discussions

with [his] expert," he was now alleging that CSA's "poor design"

contributed    to     Lawes'   accident.     Then,   in   August    2014,   CSA

participated     in    Aronberg's    eight-hour      deposition,    where    it

extensively questioned Aronberg's opinions about CSA's allegedly

negligent design.       At that time, Aronberg explained that the MOT

was "poor" (as opposed to "sloppy") because it failed to provide

clear, nondiscretionary instructions to Q.B. regarding how the

design was supposed to be implemented.          He opined, moreover, that

the design's signage placement was off, and its General Notes

needed to be more specific to comport with industry standards,

including the MUTCD (the traffic engineer's Bible).           At one point,

Aronberg was asked (by Q.B.) whether it would have been helpful

for CSA to note the real-life dimensions of the roadways in its

design,    and      Aronberg     agreed      that    would   have     helped.

He nevertheless explained that a contractor familiar with roadway

work should have been able to implement the plan without placing

the midblock barrier partially in the street, causing the eastbound

lane to fall under 8 feet in width.           In other words, although he

believed the MOT was poor (or sloppy), Q.B. should have advocated

for a blocked off narrow lane as soon as it discovered the width

issue.    When Aronberg was confronted with new material on the

Daubert stand in the form of the superimposed MOT, he still stuck


                                    - 45 -
to his guns about Q.B.'s ultimate responsibility for implementing

a pedestrian corridor (never mind CSA's sloppy design).                    Under

these circumstances, we think it improper to penalize an expert

for reacting to a defendant's evolving theories of the case. After

all, as every trial lawyer knows, "[e]vidence and theories evolve

in the last minute preparation for trial and trial itself" so "[i]t

is common for there to be some deviation between what was said in

discovery and what comes out at trial," Licciardi, 140 F.3d at

367,   especially    as   witnesses       respond   to   opposing   counsels'

suggestions on cross-examination.           At bottom, the district court

does not explain how, in view of the deposition excerpts available

to it, CSA was surprised by Aronberg's characterization of the MOT

as sloppy during the Daubert hearing.          See Gay v. Stonebridge Life

Ins. Co., 660 F.3d 58, 64 (1st Cir. 2011) (finding that the

expert's     testimony    was    a   "reasonable     elaboration"     of     his

previously    disclosed    opinions       "[a]lthough    his   testimony    uses

different words").

             The   district     court's     other   example    of   Aronberg's

surprising Daubert testimony concerned the expert's opinion that

CSA should have done more to close the sidewalk notwithstanding

the phantom bus stop.      But this too was foreseeable to anyone who

attended Aronberg's second deposition or reviewed the deposition

excerpts in the record.       Aronberg explained during his August 2014

deposition that CSA's design team should have closed the sidewalk


                                     - 46 -
at the intersection even if that meant following-up with the Bus

Authority about the bus stop.          And when (or if) CSA learned the

bus stop was not going to be moved to the southern sidewalk,

Aronberg's opinion at his deposition was that CSA had a duty to

see that the MOT was designed (or redesigned) with a closed

sidewalk based on industry standards.           The thrust of Aronberg's

opinion was the same at his Daubert hearing, even though he was

asked about new evidence (here, Vargas' trial testimony) on the

spot.     There was no meaningful difference between Aronberg's

Daubert and deposition testimony.

           In   holding   that   CSA    was   somehow    surprised   by   what

Aronberg had to say, the district court did not give any effect to

Aronberg's depositions.      The court even said that it would not

treat "hundreds of pages of deposition testimony as sufficient

notice of Aronberg's testimony."         We have no quarrel at the moment

with the district court's position that deposition testimony is

not a "suitable substitute" for a Rule 26(a)(2) expert report or

a supplemental report under Rule 26(e).             However, there is no

support in the rules or our case law for disregarding deposition

testimony in considering whether (and to what extent) sanctions

are appropriate given the discovery violations at issue.             District

courts should "consider all the circumstances surrounding [an]

alleged   [expert   disclosure]        violation"   in    considering     what

sanction (if any) is warranted in a given case.              Thibeault, 960


                                  - 47 -
F.2d at 246; see González-Rivera v. Centro Médico Del Turabo, Inc.,

931   F.3d   23,     27   (1st   Cir.   2019)   ("When    evaluating   the

appropriateness of a sanction, a reviewing court must take into

account the totality of the circumstances.").            Careful attention

to the pretrial record, in particular, is necessary to determine

whether a party's failure to abide by Rule 26's expert disclosure

requirements resulted in harm to the other party at trial.             For

instance, if the pretrial record reveals that the party opposing

sanctions provided notice of a change in its expert's testimony

(even if insufficient to satisfy the duty to supplement), then the

other side's "claimed surprise" at trial is less credible, and the

district court should consider whether a lesser sanction (if any)

is appropriate for the discovery violation.          See Licciardi, 140

F.3d at 366.       Here, the severity of Lawes' misconduct turns (in

part) on whether CSA was surprised by (and thus unprepared for)

Aronberg's Daubert testimony regarding flaws in the MOT.                To

determine whether CSA's surprise was genuine, the district court

should have reviewed and considered whether Aronberg's August 2014

deposition testimony put CSA on notice of the pertinent changes in

Aronberg's opinions regarding the design.        See Curet-Velázquez v.

ACEMLA de P.R., Inc., 656 F.3d 47, 56–57 (1st Cir. 2011) (citing

Brennan's Inc. v. Dickie Brennan & Co., 376 F.3d 356, 375 (5th

Cir. 2004) (affirming denial of a motion to exclude plaintiff's

late-filed expert report where the defense had access to the


                                  - 48 -
documents underlying the expert's opinion and where the defense

was already familiar with the underlying data)).           The district

court's disregard for deposition testimony in this case amounts to

a meaningful error in judgment that, in turn, precipitated the

district   court's   erroneous   conclusion   that   CSA   was   in   fact

surprised by Aronberg's testimony.        Because the pretrial record

does not support any claim of surprise in this case, we cannot

agree that "the punishment . . . approximately fit the crime."

Esposito, 590 F.3d at 80.

           Next, the district court's concerns about prejudice36 are

reasonable, but they still do not tip the scale in favor of the

case-dispositive     sanction   imposed   here.   The   district      court

observed, for example, that defendants would be forced to "read

hundreds of pages of [Aronberg's] depositions" in order to prepare

for trial.    While we assume trial attorneys routinely review

deposition transcripts when preparing for trial, we nevertheless

recognize that Rule 26 was designed to reduce the significant

burden of managing expert discovery. We can also reasonably assume

that Lawes' failure to supplement Aronberg's report generated

countless hours of extra work for the lawyers involved in this



     36 Although the district court determined that Aronberg's
preliminary report violated Rule 26 because it failed to disclose
his expert fee and photographs that he purportedly relied upon,
neither the court nor the defendants claim that these omissions
resulted in surprise or prejudice to defendants at trial.


                                 - 49 -
litigation.       Notwithstanding the inconvenience (and monotony) of

deposition transcript review, CSA had more than enough time (nearly

two years) before trial to prepare its defense to the opinions

Aronberg expressed in his August 2014 deposition. As we've already

explained,    there       was   no   meaningful       difference   between       his

deposition testimony and the opinions he offered at the Daubert

hearing.     Regardless, the record reflects that CSA made good use

of the time it had to prepare for Aronberg's testimony.                    Indeed,

CSA made Yates, its own traffic engineering expert, available for

a deposition a few days after Aronberg was deposed for the second

time.    Yates' deposition testimony, which referenced and at times

rebutted Aronberg's deposition testimony, suggests that CSA was

aware of Aronberg's opinions regarding its liability and was

actively preparing its defense for trial.                 Unlike the cases in

which   we   have    affirmed    a   more    severe    sanction,   there    is    no

indication here that Lawes' expert disclosure violations prevented

CSA from prepping its theory of the case for trial.                Cf. Santiago-

Díaz v. Laboratorio Cliníco y De Referencia Del Este & Sara López,

M.D., 456 F.3d 272, 277 (1st Cir. 2006) (affirming preclusion where

"plaintiff's foot-dragging in announcing her expert and providing

his report deprived the defendants of the opportunity to depose

him,    impeach     his   credentials,      pursue    countering   evidence,     or

generally prepare their defenses"); Macaulay, 321 F.3d at 52

(affirming preclusion of a supplemental expert report where the


                                      - 50 -
late-filed disclosure would have either "force[d] the defense to

trial without appropriate preparation (such as targeted pretrial

discovery)" or required the court to "reopen discovery and vacate

the trial assignment"); Licciardi, 140 F.3d at 363 (ordering a new

trial where, given the defense expert's pretrial concession that

the accident caused the plaintiff's trauma, the plaintiff had no

reason to develop "the sort of testimony which plaintiff would

have put in" had the plaintiff known before trial that defendant's

expert in fact planned to contest that the accident caused the

trauma); Thibeault, 960 F.2d at 247 ("In this case, had the court

allowed the tardy supplementation, [the defendant] would have had

to scrap much of its earlier preparation in favor of a frantic,

last-minute    scramble      to   investigate   the    emergent   witnesses,

counter their testimony, and rebut a new and different case

concept."); Freund v. Fleetwood Enters., Inc., 956 F.2d 354, 358

(1st Cir. 1992) (holding that the trial court properly excluded

plaintiff's expert testimony where substance of that testimony was

not made known to defendants until the middle of trial, and noting

that "had [defendants] known about the [expert] testimony sooner,

they   might   well   have    decided   to   counter    it,   through   cross-

examination or other expert testimony").              Thus, the record here

lacks the surprise or prejudice that warrants the "strong medicine"




                                    - 51 -
of    precluding    Lawes'   sole   expert   during   his   case-in-chief.

Esposito, 590 F.3d at 79.37

            Although we have never affirmed an expert's preclusion

when we were not persuaded by the proffered evidence of surprise

or prejudice in the record, for the sake of completeness, we'll

address the other factors relevant to our sanctions analysis.            As

to the history of the litigation, Lawes timely disclosed Aronberg's

preliminary report, and he made his expert available for two, full-

day depositions. At Aronberg's second deposition, CSA was provided

the opportunity to scrutinize Aronberg's opinions.          It even heard

directly from the horse's mouth that Aronberg did not plan to write

a    supplemental   report   regarding   CSA's   liability;    rather,   as

Aronberg told defendants, he believed his depositions adequately



       37
       Although CSA is the focus of our review on the sanctions
front (since it is the only defendant with any skin left in the
game), the district court's examples of surprise and prejudice as
to former defendant Q.B. similarly fail to justify the sanction
imposed. The district court found, for example, that Aronberg's
Daubert testimony disclosed new sources.       But these sources
(Section 6B of the MUTCD and Specification 638) were incorporated
by reference into CSA's MOT, they were disclosed in Aronberg's
preliminary report and depositions, referenced in the joint
pretrial order, and/or acknowledged by defendants' experts in
rebutting Aronberg's opinions. For instance, Q.B.'s expert, Fred
Hanscom, was deposed regarding whether Section 6B imposed
monitoring obligations on contractors, and he agreed during his
deposition that "it was a mandatory requirement for [Q.B.] to read
Specification Number 638" to understand its responsibilities.
In the same vein, contrary to the district court's assertions,
Aronberg's depositions clearly gave adequate heads-up about his
opinions regarding Lawes' path the night of his accident, as well
as Q.B.'s lighting and monitoring obligations.


                                    - 52 -
conveyed all his opinions.      CSA also signed on to a jointly filed

pretrial order, where Lawes summarized Aronberg's expert opinions

against CSA.       There is no evidence that Lawes deliberately and

repeatedly disregarded his discovery obligations.               Cf. Santiago–

Díaz, 456 F.3d at 277 & n.4 (upholding the preclusion of a late-

disclosed     expert     witness,     where      the    sanctioned      party's

"dereliction was both obvious and repeated" and "[t]he record makes

manifest    that   the   plaintiff    was     guilty   of   several   discovery

violations besides those related to her expert witness").                  The

fact that CSA cried foul for the first time during the Daubert

hearing, after the district court described one of Aronberg's

opinions as "late-arriving," further suggests Lawes' conduct and

disclosures throughout the litigation were at least minimally

sufficient to defendants until the very end.                 The next factor,

substantial justification, does not favor Lawes since he has not

offered one.       (In fact, Lawes fell on his sword, admitting his

lack of 100% compliance with Rule 26's updating requirement, but

tried to stress to the court that his failure didn't end the

inquiry.) Finally, as the district court explained, the importance

of Aronberg's testimony to Lawes' case could not be "understated."

On balance, given our review of the evidence of surprise and

prejudice identified by the district court, the history of the

litigation, and the undeniable import of the excluded testimony to

Lawes' case against CSA, we find that preclusion was overly strong


                                     - 53 -
medicine and thus, an abuse of discretion.38                We do not discount

the   district     court's   valid       concerns   about    Lawes'   discovery

violations and respect for the defendants' (and the court's) time.

Even so, in this case, we find that preclusion was excessive.                See

Enlace, 848 F.2d at 318 (finding that dismissal with prejudice

constituted an abuse of discretion and advising the district court

to consider the "broad panoply of lesser sanctions" available to

it on remand (quoting Richman v. Gen. Motors Corp., 437 F.2d 196,

199 (1st Cir. 1971))).       So we reverse the sanction imposed by the

district   court    under    Rule   26    and   Rule   37(c)(1).      To   ensure



      38In its statement of supplemental authorities filed on July
26, 2019 pursuant to Federal Rule of Appellate Procedure 28(j),
CSA argues that our decision in González-Rivera compels a different
outcome here. But that case is readily distinguishable. There,
we found the district court properly considered the totality of
the circumstances in excluding plaintiff's expert report, which
was filed nearly a year after the court's discovery deadline (not
to mention defendants' motions for summary judgment) and attempted
to revive plaintiff's claims against a defendant she had previously
moved to dismiss from the litigation. González-Rivera, 931 F.3d
at 26-28. Here, by contrast, the court chose not to consider an
entire category of documents relevant to its inquiry (i.e.,
Aronberg's depositions), and identified purported prejudice that
was unsupported by the record. The circumstances before us bear
no resemblance to those at issue in González-Rivera.            The
authorities cited in CSA's other 28(j) letters do not add anything
new to our discussion, so we won't mull over them here.         See
Aponte-Bermúdez, 944 F.3d at 964 (explaining that a claimant in a
negligent design case ordinarily must put up an expert to opine on
the applicable standard of care); Ciomber v. Coop. Plus, Inc., 527
F.3d 635, 638 (7th Cir. 2008) (affirming exclusion of plaintiff's
expert, where expert's report was filed after the district court's
discovery deadline and plaintiff failed to file a supplemental
expert report by his own self-imposed deadline).



                                     - 54 -
proceedings consistent with this opinion, we also reverse the

district court's order precluding Aronberg's orange safety fence

opinion    for   expert    disclosure    violations    during   the   Daubert

hearing.

                              Rule 702

            Lawes   also    seeks   reversal   of    the   district   court's

decision to prohibit Aronberg from testifying at trial as an expert

witness. Daubert assigns the trial court the role of "gatekeeper,"

which requires courts to make an independent determination that

"any and all scientific testimony or evidence admitted [at trial]

is not only relevant, but reliable."                Daubert v. Merrell Dow

Pharm., 509 U.S. 579, 589 (1993).39          The Daubert court concluded,

moreover, that Rule 702 displaced the "general acceptance" test of

Frye v. United States, 293 F. 1013 (D.C. Cir. 1923), under which

"the admissibility of an expert opinion or technique turned on its

'general acceptance' vel non within the scientific community."

Ruiz-Troche, 161 F.3d at 80.        Rule 702 provides that:

            A witness who is qualified as an expert by
            knowledge,   skill,    experience,   training,   or
            education may testify in the form of an opinion or
            otherwise   if:   (a)   the  expert's   scientific,
            technical, or other specialized knowledge will help
            the trier of fact to understand the evidence or to
            determine a fact in issue; (b) the testimony is
            based on sufficient facts or data; (c) the

     39 While Daubert remains relevant, Kumho Tire Co. v.
Carmichael, 526 U.S. 137, 152 (1999), clarified that the gatekeeper
function applies to all expert testimony, not just scientific.
Kumho, 526 U.S. at 141.


                                    - 55 -
              testimony is the product of reliable principles and
              methods; and (d) the expert has reliably applied
              the principles and methods to the facts of the case.

The Rule, therefore, "necessitates an inquiry into the methodology

and the basis for an expert's opinion."                   Samaan v. St. Joseph

Hosp., 670 F.3d 21, 31 (1st Cir. 2012).

              Reliability      is    a    flexible    inquiry,     allowing   for

consideration of factors like whether the expert's methodology has

been objectively tested; whether it has been subjected to peer

review and publication; the technique's known or potential error

rate;   and    whether   the    expert's      technique    has   been   generally

accepted within the relevant industry. Milward v. Acuity Specialty

Prods. Grp., Inc., 639 F.3d 11, 14 (1st Cir. 2011) (citing Daubert,

509 U.S. at 593-94).        At the end of the day, however, "[t]he focus

. . . must be solely on principles and methodology[.]"                  Daubert,

409 U.S. at 594-95.

              Notwithstanding the deep dive that courts often take to

adequately      assess   the        reliability      of   expert    methodology,

especially in highly technical industries, they must stop short of

weighing the evidence, evaluating credibility, or unnecessarily

picking sides in a battle between experts. "So long as an expert's

scientific testimony rests upon 'good grounds, based on what is

known,' it should be tested by the adversarial process."                Milward,

639 F.3d at 15 (quoting Daubert, 509 U.S. at 590).                  However, the

"reliability" bar cannot be met "by an expert's self-serving


                                         - 56 -
assertion that his conclusions were derived by the scientific

method"; rather, "the party presenting the expert must show that

the expert's findings are based on sound science, and this will

require some objective, independent validation of the expert's

methodology."    Daubert v. Merrell Dow Pharm., 43 F.3d 1311, 1317-

90 (9th Cir. 1995)("Daubert on remand").

           In addition, to be "helpful" to the jury, the expert's

conclusions   must    have   a   "valid   scientific     connection   to   the

pertinent inquiry[.]" Id. at 1320. This means that the conclusion

must not only be relevant to the facts at issue, but also that

each step in the expert's process, including the link between the

universe of pertinent facts and his conclusions, must be reliable.

Although the court must focus "on principles and methodology, not

on the conclusions they generate," Daubert, 509 U.S. at 595, this

focus "need not completely [preclude] judicial consideration of an

expert's conclusions."       Ruiz-Troche, 161 F.3d at 81.        In General

Electric   Co.   v.   Joiner,    the   Supreme   Court   acknowledged      that

"conclusions and methodology are not entirely distinct from one

another.   Trained experts commonly extrapolate from existing data.

But nothing in either Daubert or the Federal Rules of Evidence

requires a district court to admit opinion evidence that is

connected to existing data only by the ipse dixit of the expert."

522 U.S. 136, 146 (1999). For this reason, "[a] court may conclude

that there is simply too great an analytical gap between the data


                                   - 57 -
and the opinion proffered."              Id.; see Samaan, 670 F.3d at 32

(providing that the trial court may "examin[e] . . . [the expert's]

conclusions to determine whether they flow rationally from the

methodology employed").          An "analytical gap between the data and

the opinion proffered" may provide the basis for the expert's

exclusion.      Samaan, 670 F.3d at 32 (quoting Gen. Elec. Co., 522

U.S. at 146).      This requirement, which is sometimes described as

"fit," ensures that the connection between the expert's data, his

conclusions, and the facts of the case is reliable.               See id.

              The Daubert inquiry is case-specific.           "Exactly what is

involved in 'reliability' . . . 'must be tied to the facts of a

particular case.'"       Milward, 639 F.3d at 14–15 (quoting Beaudette

v. Louisville Ladder, Inc., 462 F.3d 22, 25 (1st Cir. 2006)).

Adding to the complexity, "there is no particular procedure that

the trial court is required to follow in executing its gatekeeping

function under Daubert."            United States v. Diaz, 300 F.3d 66, 73

(1st Cir. 2002).        Importantly, "Daubert does not require that a

party who proffers expert testimony carry the burden of proving to

the   judge    that    the    expert's   assessment   of    the   situation   is

correct"; rather, to satisfy Daubert's objective, the proponent

must show "that the expert's conclusion has been arrived at in a

scientifically        sound   and    methodologically      reliable   fashion."

Milward, 639 F.3d at 15 (quoting Ruiz-Troche, 163 F.3d at 85).

"Vigorous cross examination, presentation of contrary evidence,


                                       - 58 -
and careful instruction on the burden of proof are the traditional

and appropriate means of attacking shaky but admissible evidence."

Daubert, 509 U.S. at 596.

              In   this   case,   the   district   court   began   Aronberg's

Daubert hearing on its own initiative toward the end of the

plaintiff's case-in-chief.         Aronberg, the only witness to testify

at the hearing, was cross-examined by multiple defense counsel for

twelve days.        The trial court admitted and considered evidence

solely for the purpose of the Daubert hearing, cross-examined the

witness, and fielded countless spats between the parties during

and   after    long   hearing     days.    Following   this   grueling   and

acrimonious procedure, the trial court refused to qualify Aronberg

as an expert in Lawes' case, concluding:           (1) Aronberg's opinions

were not supported by sufficient data; (2) Aronberg's methodology

was inconsistent with one article written by another expert in the

field and was therefore unreliable; and (3) Aronberg did not

reliably apply the principles and methods of traffic engineering

to the facts of Lawes' case in reaching his expert opinions. After

careful review and consideration of our deferential approach, we

find that Aronberg's opinions -- although not bulletproof -- were

sufficiently reliable to present to a jury. By excluding Aronberg,

"the district court exercised its gatekeeping role under Daubert

with too much vigor."       Manpower, Inc. v. Ins. Co. of Pa., 732 F.3d

796, 805 (7th Cir. 2013).         Our reasoning follows.


                                    - 59 -
                         Sufficiency of Aronberg's Data

              Rule 702 requires that expert testimony be based on

"sufficient facts or data."           Fed. R. Evid. 702(b).      Although an

expert's methodology is the "central focus of a Daubert inquiry,"

courts "may evaluate the data offered to support an expert's

bottom-line opinions to determine if that data provides adequate

support to mark the expert's testimony as reliable."             Ruiz–Troche,

161    F.3d   at   81.     However,   district   courts   must   not   "unduly

scrutinize[] the quality of the expert's data," because such

scrutiny "usurps the role of the jury."             Manpower, 732 F.3d at

806.

              To begin, the district court proclaimed that "[n]either

case law nor the Rules require courts to scrutinize the sufficiency

of [an expert's] data."        The court nevertheless raked a fine-tooth

comb through Aronberg's sources and concluded the expert had missed

certain data that it considered necessary to "lend proper support"

to his opinions.         The district court found that Aronberg failed to

review "any contractual documents pertaining to the Bahía Urbana

[construction] project" or any Puerto Rico law.              These sources,

according to the district court, were relevant to understanding

the roles and responsibilities of the parties to the Project

contract.      Because Aronberg did not consult these sources, the

district court further determined that Aronberg's conclusions

regarding the legal and contractual duties owed to pedestrians


                                      - 60 -
like Lawes were not supported by sufficient data.                     In particular,

the district court observed that Aronberg's lighting opinion --

i.e., that Q.B. (in its role as primary contractor for the Project)

was responsible for and failed to provide adequate illumination of

the   construction-affected           area     –-    was     based    on    Aronberg's

misunderstanding of Q.B.'s obligations.40                  As the district court

explained, if Aronberg had reviewed the construction project plans

and/or     any   local    law,   he    would    have    known    that      third-party

defendant     PREPA   (and    not     Q.B.)    was   responsible        for   ensuring

streetlights near the scene of Lawes' accident were functioning

properly.41

             The district court overstates the importance of these

unspecified "contractual documents" and local law to Aronberg's

expert opinions.         We favorably assume that portions of the Bahía

Urbana     construction      project    contract       set    forth    the    parties'

responsibilities for managing the flow of traffic in the area


      40The district court also determined that Aronberg was
unfamiliar with CSA's role in the Project because the expert
described the MOT as sloppy "for the first time" during the Daubert
hearing. As we noted earlier, however, Aronberg opined on CSA's
role as MOT designer and the purported flaws in the MOT during his
second deposition in August 2014 based upon the sources available
to him at the time. See supra at 42-43.
      41CSA likewise claims that Aronberg's conclusions were not
based on sufficient data since the expert failed to review relevant
contract documents and, instead, relied upon his experience in the
industry to determine whether the parties breached their
contractual obligations.



                                       - 61 -
covered by the MOT.42     And we recognize that a review of local law

could prove useful for understanding municipal authority in and

around roadways (like Fernández Juncos).       But even if we assume

(as the district court did) that Aronberg did not review these

sources,43   there   is     no   indication   that   Aronberg's   data

insufficiently supported his opinions regarding "who's who" and

"who's responsible for what." Aronberg considered the MUTCD, which

even defendants' experts described as "clearly delineat[ing] the

responsibilities of who does what in terms of what [the parties

were] responsible for."     He also reviewed streetlight maintenance

records produced by PREPA, who was responsible for conducting

streetlight maintenance in the area of Lawes' accident.            As

Aronberg gained access to new documents during discovery, he

modified his expert opinions and offered new opinions during



     42CSA does not dispute that its responsibility was to design
the MOT. And also relevant to our analysis, Q.B.'s responsibility
was to implement the MOT design, abiding by its General Notes.
Q.B. did not argue otherwise before the district court.
     43The district court does not point us to the sections of the
contract documents that it believes were relevant; nor does it
identify Aronberg's testimony that he did not in fact review them.
CSA tells us to read Aronberg's testimony on Day 5 of the Daubert
hearing. But there, Aronberg admitted that he had not read "CSA's
contract," which is related to but not entirely consistent with
the district court's finding that Aronberg "did not mention
analyzing any contractual documents pertaining to the [Project]."
Lawes, however, does not challenge the district court's assessment
and, instead, concedes "Aronberg's non-reliance on the contract -
- outside of the MOT and its General Notes, the MUTCD and
Specification 638 . . . ."


                                 - 62 -
depositions    based     on    his   more    complete    understanding       of   the

parties' contractual duties and their purported breach of them.

Between    publishing         his    preliminary      report    and    his    first

deposition, for example, Aronberg was able to review the complete

MOT, including the notes from CSA with specific instructions and

relevant     standards        that   Q.B.     was    required     to   implement.

Thereafter, Aronberg reviewed hundreds of pages of deposition

testimony, during which parties to the contract explained their

roles in (among other things) the underlying construction Project,

the management of traffic in the construction-affected area, and

their duty to ensure the safety of pedestrians traversing the

construction-affected area.            Aronberg considered expert reports

produced by defendants, which further defined the key players'

obligations and often rebutted Aronberg's opinions as to the

players' liabilities.           Aronberg's data likewise included pre-

litigation    documents       produced      from    Project    files   (e.g.,     the

meeting minutes and RFIs that covered day-to-day action items and

objectives for each party to the contract).                    Taken as a whole,

these documents provide sufficient data from which Aronberg could

reliably opine about the parties' respective obligations to keep

the area safe for pedestrians.                The district court's concern,

therefore, would be valid only "if the parts of the record that

[Aronberg] did not read contained information that was unavailable

in the parts that he did read."              Mitchell v. United States, 141


                                       - 63 -
F.3d 8, 16 (1st Cir. 1998).           Rule 702 does not demand that experts

rely on all data that could be deemed relevant.                   It does not even

require the expert to seek out the best possible source of relevant

information.      "Sufficien[cy]" is the benchmark for an expert's

data under the Rule.          Fed. R. Evid. 702(b); see 29 Charles Alan

Wright & Arthur R. Miller, Federal Practice and Procedure § 6268

(2d ed. 2017) (explaining that "the word 'sufficient' in Rule 702

signifies      that   the    expert   may    properly    base     her    opinion     on

something less than all the pertinent facts or data" and, as a

result, "sufficiency is not a matter of whether the judge believes

in the facts or data on which the expert relies").                      The district

court erred by holding Aronberg's data to a more stringent standard

than was required by the rules.

            Relatedly,       we   take    issue   with   the    district       court's

characterization of Aronberg's lighting opinion, which was offered

by the court as an example of how the expert's insufficient data

failed   him    during      the   Daubert    hearing.       The    district     court

determined that the lighting opinion was supported by Aronberg's

incorrect assumption that Q.B. had a duty to provide adequate

lighting in the area when, in fact, PREPA was the only entity

responsible for streetlights.             Based on our review of the record,

and as Lawes' counsel attempted to explain during the Daubert

hearing,    however,        Aronberg's      opinion   was      that     Q.B.   had   a

contractual duty to provide lighting (temporary or otherwise) in


                                         - 64 -
the construction area, including the northern sidewalk.            During

his first deposition, for example, Aronberg explained: "Q.B. would

have    a   duty   to   provide   alternative   illumination    once    the

[street]lights [were] turned off" by PREPA.           He also explained

that his lighting opinion was supported by his review of the

complete MOT44 and Vargas' deposition testimony regarding Q.B.'s

contractual duty to illuminate the work area.           Accordingly, the

district court has not identified anything that would suggest to

us that Aronberg's lighting opinion was based on insufficient data.

As best we can discern, the court's issue with Aronberg's data was

that it could have been more robust and not, as the court claimed,

that it was insufficient as a whole.          For reliability purposes,

however, the court's evaluation of the data must be limited to

determining whether it "provides adequate support to mark the

expert's testimony as reliable."           Ruiz–Troche, 161 F.3d at 81;

Packgen v. Berry Plastics Corp., 847 F.3d 80, 88 (1st Cir. 2017)

(affirming    admission   of   expert   testimony   where   expert's   data

"provided the minimal basis necessary to support" the assumptions


       44
       Although Aronberg was not asked to identify the section of
the MOT that supported his lighting opinion, he nevertheless
explained during his November 2013 deposition that "[the MOT] says
the contractor shall provide adequate illumination in the work
area at all times." His testimony was consistent with MOT General
Note 9, which states:     "the contractor shall provide adequate
illumination in the work area at all times."     Moreover, Lawes'
portion of the joint pretrial report explains that "defendants
failed to comply with General Note No. 9 which states that Q.B.
had to provide artificial illumination at all times."


                                  - 65 -
underlying expert's conclusions, even though the expert could have

done a market survey to test them further).

            In    sum,   while   an    "expert    opinion   grounded    on   a

nonexistent fact is not significantly probative," Borges ex rel.

S.M.B.W. v. Serrano-Isern, 605 F.3d 1, 8 (1st Cir. 2010), the

district court points to no authority that supports its conclusion

that Aronberg's testimony was unreliable merely because there were

perhaps other relevant sources he did not consider.              Even if it is

good practice for an expert to review all available contract

documents and applicable law, whether Aronberg took shortcuts in

his data collection efforts "is a matter affecting the weight and

credibility of the testimony — a question to be resolved by the

jury."   United States v. Vargas, 471 F.3d 255, 264 (1st Cir. 2006)

(quoting Int'l Adhesive Coating Co. v. Bolton Emerson Int'l, Inc.,

851 F.2d 540, 545 (1st Cir. 1988)).              In our view, the district

court placed undue weight on the sources Aronberg did not consider,

which contained facts that were readily available to Aronberg as

part   of   the   data   he   did     consider.     Even    if   the   factual

underpinnings of Aronberg's opinions could be viewed as weaker

than they would have been had he considered the data the court

focused on, "that was a matter affecting the weight and credibility

of [his] testimony," not its admissibility. Payton v. Abbott Labs,




                                      - 66 -
780 F.2d 147, 156 (1st Cir. 1985) (citing Coleman v. DeMinico, 730

F.2d 42, 47 (1st Cir. 1984)).

              Reliability of Aronberg's Methodology

          Aronberg's   methodology       appears    to   us   relatively

straightforward:   (1) he conducted an investigation into the

conditions in place on the night of Lawes' accident, including by

visiting the scene on two occasions, reviewing the police accident

report, developing a computer automation of the accident, and

reviewing photos, testimony, the MOT and underlying documents, and

other sources; and (2) he applied the relevant sections of the

MUTCD (and other standards) to the facts ascertained from his

investigation to determine what conditions should have been in

place to protect pedestrians from injury.          Aronberg opined that

his methodology was generally accepted in the field of traffic

engineering, which hails the MUTCD as the "Bible." Defendant CSA's

expert traffic engineer, Yates, applied a similar methodology to

rebut Aronberg's opinions.    Aronberg explained all of this again

during his Daubert hearing.     The district court still took issue

with Aronberg's methodology, which diverged from recommendations




                                - 67 -
set forth in one article by Himat Chadda and Thomas E. Mullnazzi

that was published in the ITE in 1987 (the "1987 Chadda article").45

             Here's the run down.              On Day 9 of Aronberg's Daubert

hearing, Q.B.'s counsel took Aronberg to task on the minutia of

the expert's methodology. He asked Aronberg to confirm the sources

he had relied upon and why, and then asked Aronberg whether he

conducted a number of tests (discussed in more detail later) which

Aronberg confirmed he had not conducted because they were not

germane     to   his   opinions.        Counsel       then   directed    Aronberg's

attention to the 1987 Chadda article, which the district court

allowed into evidence for purpose of the Daubert hearing only.46

The   article    is    entitled    "The    Traffic      Engineer    as   an    Expert

Witness."        The   purpose     of     the     article,    according       to   its

introduction, is to "discuss[] how a traffic engineer can prepare

for the challenging role of an expert" and to provide an "outline"

of    the   "various    types     of    data    and   information    needed,       the

documentation and evaluation of the data, and the presentation of



       45
       For context, the ITE is published by an organization that
Aronberg previously described as the "most widely recognized
organization for transportation engineers in the world." Chadda
is a professional engineer whom Aronberg was generally familiar
with from his years in the industry. Aronberg even brought one of
Chadda's articles from 1984 to his second deposition to support
testimony regarding pedestrian behavior in a construction zone.
       46
        Although Q.B.'s counsel admitted the 1987 Chadda article
into the Daubert record, he did not explain how he came by the
article and it does not appear to be part of the pretrial record.


                                        - 68 -
the data for expert testimony."               The article also includes a list

of     "dos   and     don'ts     .    .   .   that    should     help    improve    the

effectiveness" of traffic engineers serving as expert witnesses.

In essence, the article contains what could be described as best

practices      for    traffic        engineers      testifying    as    experts,    and

implicitly recognizes that such practices will vary based on the

facts of each case.

              As was relevant to the district court, the article

recommends that expert witnesses carefully study the statements

from    accident      witnesses,       take   into    consideration      that   police

accident reports may be inaccurate, visit the accident around the

time and day similar to the accident, and conduct certain studies

designed to measure the volume of traffic on the roadway where the

accident      at    issue     occurred.       The    district    court    found    that

Aronberg's          Daubert      testimony         regarding     his      methodology

contradicted "Chadda's methodology."                    Namely, as the district

court tells it, Aronberg did not consider it important to interview

eyewitnesses to the accident and, instead, he testified that the

police accident report was the more important document.                      Relevant

to the district court, the 1987 Chadda article describes police

reports as the "weak link" in the traffic engineering expert's

investigation.         The district court also dinged Aronberg for not

performing         certain     traffic    engineering      studies       despite    the




                                          - 69 -
article's recommendation that experts conduct any such studies

that are "appropriate" for their case.

             As a threshold matter, it is not clear to us why, based

on the record here, the 1987 Chadda article should be viewed as

providing the definitive methodology for traffic engineers serving

as expert witnesses, and the district court never explains its

willingness to place so much reliance on it. Indeed, the article's

introduction describes what follows as a set of recommendations

designed     to   increase     the    "effectiveness"        (as   opposed   to

reliability) of rendering expert witness testimony, and warns

experts against "embarrassment" at trial (rather than exclusion

under Rule 702).        But even if the record established the 1987

Chadda article as an authoritative source on traffic engineering

methodology, Aronberg's investigation complied with many of the

article's recommendations.           For instance, consistent with the

article and contrary to the district court's assessment, Aronberg

visited the accident site on two occasions, including once at

night. The article also encourages the use of technical references

like   the   MUTCD    and    guidance      published    by   AASHTO   (American

Association of State Highway and Transportation Officials) -- both

of which were cited by Aronberg as part of his methodology and

data   considered.       Moreover,      the   alleged    differences    between

Aronberg's    methodology     and    the    article's    recommendations     are

insignificant.       Contrary to the district court's assertions, the


                                     - 70 -
transcript of Aronberg's Daubert testimony does not indicate that

the expert viewed fact witness statements as unimportant or less

important than police reports.          Rather, Aronberg testified that it

was not important to talk to each witness that could have possibly

observed the accident.          Later that same day, Aronberg testified

that he received and considered a summary of eyewitness accounts

of the accident from Lawes' counsel prior to drafting his report.

Aronberg then provided a step-by-step retelling of the events

immediately prior to Lawes' accident from the perspective of Lawes'

shipmate and fellow jaywalker, Gordon.         Neither the defendants nor

the   court    averred   then    that   Aronberg's   understanding   of   the

accident was incomplete or otherwise unreliable.           With respect to

police accident reports, Aronberg merely opined that he had relied

on such reports in other cases and did so again here.                Without

more, the district court has not provided any support for its

conclusion that Aronberg's consideration of witness statements and

the police accident report here was unreliable.

              The district court also found Aronberg's methodology

unreliable because he did not conduct certain traffic studies the

court believed necessary.         The 1987 Chadda article proposes that

experts conduct "appropriate studies," including a "speed study"

and   a   "complete   collision    diagram."      Extrapolating   from    the

article's study-based recommendations, the district court found

that Aronberg did not perform a "vehicle capacity study" or a "peak


                                    - 71 -
pedestrian/vehicular volume study."                 But these studies are not

mentioned at all in the 1987 Chadda article.                   They were brought up

by Q.B.'s counsel during cross-examination at the Daubert hearing,

and several days later Q.B. referenced Aronberg's failure to

conduct    these     studies     in    its    motion     to    exclude   Aronberg's

testimony.        In apparent agreement with Q.B., the district court,

pointing to nothing authoritative, claimed that (without these

studies) Aronberg could not reliably assign Fernández Juncos a

roadway category which, in turn, was important to determining how

wide its lanes should be.         Recall that the appropriate and actual

width of the southernmost eastbound lane served as the factual

underpinning for Aronberg's pedestrian corridor opinion.

                However,   the   mere       "existence    of    other    methods   of

gathering facts does not mean that the facts [Aronberg] relied

upon     were    insufficient";       nor    does   it   mean     that   Aronberg's

methodology was unreliable.           Packgen, 847 F.3d at 86–87.           In lieu

of performing the capacity and volume studies that the district

court believed were necessary, Aronberg personally measured the

width of the southernmost eastbound lane during a site visit, and

he concluded that it was too narrow in view of industry guidance

concerning the required width of highway lanes.                     Like Aronberg,

Yates (CSA's expert) testified at his deposition that the MUTCD

and AASHTO provide national standards and references for highway

width.    Yates stated that "assuming" Aronberg's measurement of the


                                        - 72 -
lane was accurate, Yates ("as an engineer") would have a "problem"

with the fact that the lane was not closed.47             When asked whether

speed limits or daily volume of traffic dictated the minimum width

of   a        highway,   Yates   acknowledged    that   these    were   relevant

considerations.          However, he noted that pursuant to the MUTCD and

AASHTO the minimum acceptable width for any highway lane was nine

feet.         Considering Yates' deposition testimony, even if capacity

and volume studies are helpful for determining the appropriate

width of a given highway, no capacity study was necessary to

support Aronberg's conclusion that Fernández Juncos' southernmost

eastbound lane was narrow enough to be considered for closure (even

if such closure was not mandatory).             With no explanation as to why

it   believed       these   studies   indispensable,    the     court   erred   in

treating Aronberg's decision not to conduct them as a crucial flaw

in his methodology, especially since Aronberg's methodology was

consistent with the devices employed by other experts in the field

and logically flowed from what was known to him.                See Packgen, 847

F.3d at 88 ("Experts may, however, make reasonable assumptions

that are consistent with the evidence available to them." (citing


         47
        When asked why the "eight feet measurement of a lane or
width of a lane" would be a problem, Yates explained: "It would
be difficult for vehicles to pass through an eight foot window
that close to a wall without driving – without the drivers tending
to straddle the lane line." For this very reason and consistent
with AASHTO guidance, Aronberg concluded that the southernmost
eastbound lane could have been closed to vehicular traffic and
converted into a pedestrian corridor.


                                      - 73 -
Cummings v. Standard Register Co., 265 F.3d 56, 65 (1st Cir. 2001)

(affirming      admission     of    damage   expert's    testimony      given    the

expert's assumptions were also made by similar experts "with some

frequency")).48

               Even if the district court had identified meaningful

differences       between     Aronberg's        methodology    and      what     was

recommended in the 1987 Chadda article, "Daubert does not require

that the party who proffers expert testimony carry the burden of

proving to the judge that the expert's assessment of the situation

is correct.       It demands only that the proponent of the evidence

show    that    the   expert's     conclusion    has   been   arrived    at     in   a

scientifically        sound   and    methodologically     reliable      fashion."

United States v. Mooney, 315 F.3d 54, 63 (1st Cir. 2002) (quoting

Ruiz-Troche, 161 F.3d at 85).            In Ruiz-Troche, for example, the

defendant's expert analyzed the concentration of cocaine detected


       48
        For the first time on appeal, defendant CSA claims that
Aronberg's methodology was unreliable because he failed to perform
a visibility analysis of the driver, a perception-reaction
analysis, or a lighting analysis for the vehicle's headlights.
These studies were not mentioned in the 1987 Chadda article or the
district court's opinion.    In fact, the only mention of these
studies appears to have been during the deposition of driver
Riviere-Andino's accident reconstruction expert, Baigés Valentín.
The expert claimed to have conducted these studies as part of his
analysis, which focused on Riviere-Andino's liability for Lawes'
accident. But these studies were not germane to Aronberg's traffic
engineering expert opinions. As Aronberg explained to the court
during his Daubert hearing, he was not testifying as an expert in
accident reconstruction in this case (although he had done so in
other cases) and therefore he did not need to apply accident
reconstruction methodology to support his conclusions.


                                      - 74 -
in the body of one of the plaintiffs after a car accident to

conclude that she'd ingested the drug in a dosage that would have

impaired her perception, reflexes, reaction time, and judgment and

adversely affected her ability to drive just before the accident.

In response, that plaintiff offered peer review literature that

"cast doubt" on the expert's methodology and conclusion (for

example, they suggested his technique had "an uncertain rate of

error").   Ruiz-Troche, 161 F.3d at 85; see also id. at 86 (noting

that other literature suggested that scientists could not always

"correlate particular impairments to cocaine concentrations within

the body," and the plaintiffs urged that "only an immediate

neurological examination could have provided sufficiently reliable

evidence as to whether [the plaintiff] suffered from cocaine

intoxication   at    the   time   of   the    accident . . . and    no    such

examination    was   performed").        We   concluded   there    that    the

defendants' expert's methodology was nevertheless reliable given

that it satisfied other indicia of reliability; it still received

"significant   support     in   the    relevant   universe   of   scientific

literature" and from the relevant plaintiff's own expert.             Id. at

85.   Though the literature did not "irrefutably prove" the expert

right, it showed his methods (and therefore his testimony) was

sufficiently reliable to reach the jury.              Id. at 86.         So we




                                   - 75 -
concluded there that the district court abused its discretion in

excluding the testimony.           Id.

             Here,   as    we've    explained,    Yates   was    on   board   with

Aronberg's methodology.        He described the MUTCD as "the standard"

used "by all traffic engineers."             In fact, all the experts in the

case relied on AASHTO and/or the MUTCD in forming their opinions.

So, as in Ruiz-Troche, Aronberg's reliance on these technical

sources has been "subjected to, and survived, the rigors of

testing, publication, and peer review, and it appears to have won

significant (if not universal) acceptance within the scientific

community."    Id. at 85.

             We therefore find that the district court placed undue

weight on the importance of capacity and volume studies since

"Daubert neither requires nor empowers trial courts to determine

which   of   several      competing      scientific   theories   has   the    best

provenance."     Id.      One more time (it's worth repeating): Daubert

"demands only that the proponent of the evidence show that the

expert's conclusion has been arrived at in a scientifically sound

and methodologically reliable fashion."               Id. (citing Kannankeril

v. Terminix Int'l, Inc., 128 F.3d 802, 806 (3d Cir. 1997); In re

Paoli R.R. Yard PCB Litig., 35 F.3d 717, 744 (3d Cir. 1994)).

As was the case in Ruiz-Troche, the district erred in weighing the




                                         - 76 -
factors relevant to the reliability of Aronberg's methodology.

Id.

                  Application of Aronberg's Methodology

            In excluding Aronberg's testimony, the district court

also   identified    several      issues       with    the   application       of   his

methodology.      Although       most    of    these    issues       concern   former

defendant Q.B.'s liability, we must address the district court's

perceived   weaknesses      in    Aronberg's       testimony     to     explain     and

contextualize our analysis and ultimate determinations relevant to

CSA.   For the reasons that follow, we conclude that the district

court's concerns do not implicate "the reliability of [Aronberg's]

methodology" but (instead) constitute improper scrutiny of "the

conclusions that it generated."               Manpower, 732 F.3d at 807.

            The    Monitoring      Opinion.           The    trial    judge    deemed

unreliable Aronberg's testimony that Q.B. should have monitored

the construction site and taken appropriate steps to mitigate any

unsafe conditions observed therein.                   Here, the district court

opined that Aronberg had assumed certain facts not in evidence,

creating an analytical gap between the data Aronberg presented and

the    conclusion    that    he     drew       regarding      Q.B.'s        monitoring

responsibilities.     Of particular interest to the court, there was

testimony   at    trial   that     sailors       traveling     to     and    from   the

waterfront when the MOT was in place traveled in "scattered groups"

and took other routes that did not involve jaywalking (like


                                        - 77 -
scurrying around the midblock barrier).         Based on this testimony,

the court asserted that Aronberg's "categorical" assertion that a

monitoring plan would have detected the midblock crossing problem

had little support in the record.

           In reaching this conclusion, the district court failed

to mention that Gordon (and other sailors) testified at trial that

they   regularly   jaywalked     across    Fernández   Juncos   after    the

midblock barrier was in place in order to avoid the "hot corner"

on the northern sidewalk at the end of the pedestrian crosswalk.

Although   the     sailors     admitted    to   skirting    the    barrier

occasionally, Gordon explicitly acknowledged that the group's

preferred route was to cross at midblock rather than skirting the

barrier.   He perceived the latter as more dangerous after nearly

being "run down" during a prior skirting attempt. Gordon testified

that after the barrier was implemented (and before Lawes' accident)

he and crew members crossed at midblock (walking to and from Old

San Juan) at least once a week for over a year.         When viewed as a

whole, the sailors' testimony establishes that at least some

pedestrians developed a noticeable pattern of crossing Fernández

Juncos at midblock.          Whether this testimony establishes that

midblock crossings occurred often enough to have been observed by

a   contractor   monitoring    the   construction-affected      area    is   a

question that goes to the weight rather than the admissibility of

Aronberg's opinions about Q.B.       The district court was not free to


                                  - 78 -
take sides on disputed questions of fact in the trial record.             We

find that Aronberg's conclusions regarding Q.B.'s duty to monitor

the   area      satisfy     Daubert's     threshold      requirements    for

admissibility.

             Pedestrian Corridor, Positive Guidance, and Sidewalk

Opinions.     The district court characterized as mere speculation

Aronberg's conclusions about defendants' duties to design and/or

implement a pedestrian corridor, an orange safety fence along the

northern sidewalk, and a closed-off southern sidewalk.           In support

of this contention, the court declared that these opinions were

not practical because CSA and/or Q.B. would need to get the

approval of the Puerto Rico Highway and Transportation Authority

before closing a lane of traffic to build the pedestrian corridor,

erecting an orange safety fence along the northern sidewalk, or

changing     the   MOT    design   and   closing   the   sidewalk   at   the

intersection.       The district court's reasoning is based on a

fundamental misunderstanding of its gatekeeping function.

             From our vantage, Aronberg's opinions logically flow

from the facts in this case and reflect the reliable application

of his MUTCD-based methodology.              In view of applicable MUTCD

guidance (which, again, the district court does not identify as

the source of Aronberg's unreliability), Aronberg determined that

other traffic control devices were safer for pedestrians than the

midblock barrier, including closing the southern sidewalk at the


                                    - 79 -
crosswalk    and/or   creating   a     pedestrian     corridor    out    of   the

southernmost eastbound lane. Based on the statements and testimony

of sailors who regularly traversed the construction-affected area,

Aronberg concluded that the midblock barrier encouraged dangerous

pedestrian behavior (like jaywalking), which Q.B. should have

discovered while monitoring the area.            Adding to this, Aronberg

opined that there was no justifiable reason for the midblock

barrier to remain in place considering the dangers it posed to

pedestrians and the fact that the planned bus stop was never

relocated to the southern sidewalk.             Considering the pertinent

facts, the MUTCD, and his experience and education as a traffic

engineer, Aronberg therefore concluded that CSA and Q.B. were

obligated to take affirmative steps to remove the unsafe condition

and implement better traffic control devices.                  In particular,

according    to    Aronberg,   these    defendants     should    have    sought

permission to close the southern sidewalk at the crosswalk and (in

the case of Q.B. only) to convert the narrow lane of traffic into

a pedestrian corridor.         CSA does not argue (and there is no

evidence in the record) that it was precluded from requesting

permission    to   make   changes    to   the   MOT    after     the    design's

implementation.     There is a closer question of whether defendants'

duty to get input from municipal authorities weakens the causal

connection between their conduct and Lawes' accident. The district

court, for its part, suggests the relevant authorities would have


                                    - 80 -
denied any requests to change the MOT.        Specifically, based on the

district court's understanding of Vargas' testimony at trial, the

Puerto   Rico   Highway    and    Transportation   Authority       previously

rejected a proposal to close the southernmost eastbound lane and

requested notice of contemplated lane closures prior to the MOT's

implementation.      In light of the Highway Authority's position on

lane closures prior to the MOT, the district court speculated that

any follow-up requests to change the construction-affected area to

accommodate new traffic control devices would likely be rejected.

Although the district court did not cite to any evidence in the

record to support its theory, one could argue that municipal

authority over the area undermines Aronberg's conclusion that

defendants proximately caused Lawes' accident.            However, for our

purposes,   "there    is   an   important   difference    between    what   is

unreliable support [for an expert's conclusions] and what a trier

of fact may conclude is insufficient support for an [expert's

conclusion]."     Milward, 639 F.3d at 22.          Here, at best, the

district court's concerns involve the latter.             Accordingly, the

alleged weaknesses identified by the court go to the weight of

Aronberg's opinions, not their admissibility.            See id.

            The district court's reasoning as to the speculative

nature of the orange safety fence is even more questionable given

that the MOT expressly contemplates such a fence in General Note

11.   Neither the conclusions Aronberg reached nor the known facts


                                   - 81 -
fall outside the boundary of reliability.          At any rate, "vigorous

cross examination" and the "presentation of contrary evidence" are

the   appropriate     means    of    "attacking   shaky     but   admissible

evidence."     Daubert, 509 U.S. at 596; see also Grp. Health Plan,

Inc. v. Philip Morris USA, Inc., 344 F.3d 753, 760 (8th Cir. 2003)

(stating that "[a] certain amount of speculation is necessary,

[and] an even greater amount is permissible (and goes to the weight

of the testimony)," even if "too much is fatal to admission").

             Midblock Barrier Opinion.         According to the district

court, there is "no evidence on the record that supports the

conclusion that the placement of the barrier had any bearing" on

Lawes' accident.      The district court, therefore, ruled that the

location of the barrier, and Aronberg's related conclusions about

CSA's liability, "lack any support, do[] not fit the facts of the

case, and [are] irrelevant to the tort in question."                To reach

this conclusion, the district court either overlooked or deemed

not credible:     (1) MUTCD guidance about pedestrian behavior and

the importance of implementing traffic control devices to prevent

pedestrians    from   taking   a    shortcut   through    the   highway;   (2)

testimony from sailors explaining that their habitual route to and

from the waterfront involved crossing at midblock (both ways); (3)

testimony from sailors about their desire to cross the roadways

before the intersection at the end of the northern sidewalk, where

the "hot corner" existed; (4) testimony from CSA's MOT designer


                                    - 82 -
that (but for the phantom bus stop) she believed closing the

sidewalk at the crosswalk was the safer design; and (5) Aronberg's

training and expertise in traffic engineering.    Regardless of how

the district court got there, the result is the same. The district

court erred in applying the standard for admissibility for expert

testimony to the relevant factors in this case.    Where, as here,

the expert's methodology, his conclusions, and the facts of the

case have a "valid scientific connection to the pertinent inquiry,"

they should be submitted to the jury to aid in its deliberation.

See Daubert on remand, 43 F.3d at 1320.

          Aronberg's Contradictions on the Stand.     Finally, the

district court concluded that Aronberg's self-contradictions over

the course of his rigorous twelve-day Daubert hearing also rendered

him unreliable to serve as an expert witness at trial.      Namely,

the district court found that Aronberg testified inconsistently

about the impact of the southernmost eastbound lane on Lawes'

accident (focusing on Aronberg's testimony that the implementation

was Q.B.'s fault even if the MOT was sloppy), and that Aronberg

could not remember when he had mentioned Specification 638 during

his deposition two years prior.    The district court also found

that Aronberg's memory was shaky as to his reliance on the police

accident report and review of contract documents, and as to whether

the lighting opinion was in his original report (which he did not




                              - 83 -
have in front of him for this line of questioning).49                Moreover,

to   the      district      court's    chagrin,     after   relentless   cross-

examination over the course of several days about the phantom bus

stop,        there   were     purportedly        differences   in   Aronberg's

explanation. All of the district court's hang ups about Aronberg's

preciseness, consistency, and credibility go toward (you guessed

it) the weight and not the admissibility of his testimony.                  See

Milward, 639 F.3d at 22 ("Lack of certainty is not, for a qualified

expert, the same thing as guesswork." (quoting Primiano v. Cook,

598 F.3d 558, 565 (9th Cir. 2010)); Int'l Adhesive Coating, 851

F.2d at 545 ("When the factual underpinning of an expert's opinion

is weak, it is a matter affecting the weight and credibility of

the testimony — a question to be resolved by the jury.").

               With no further ado, we conclude that the judge "crossed

the boundary between gatekeeper and trier of fact" in this case.

Milward, 639 F.3d at 22.              Because the district court's concerns

should have been reserved for the adversarial process at trial, we



        49
        The district court also determined Aronberg's lighting
opinion was unreliable in view of Vargas' trial testimony.
However, Vargas admitted on the stand that she did "not know for
a fact who's responsible" for illumination in the event a
streetlight was malfunctioning (as was alleged to be the case the
night of Lawes' accident). Given the totality of the witnesses'
testimony, we cannot agree that it undermines the reliability of
Aronberg's opinion (based on MOT General Note 9) that Q.B. was
obligated to ensure adequate illumination of the work area
(including along the northern sidewalk).



                                        - 84 -
conclude the district court abused its discretion in excluding

Aronberg's testimony under Rule 702.50

                               WRAP UP

          We reverse the district court's rulings under Rule 26

and Rule 702, vacate the entry of judgment, and remand this matter

for further proceedings consistent with this opinion.       Costs

awarded to Lawes.

                    - Concurring Opinion Follows -




     50 To the extent the district court also suggests that
Aronberg's testimony is inadmissible under Rule 403's less
exacting relevance standard, we conclude (for all the reasons
already stated) that the probative value of the expert's testimony
to Lawes' case outweighs any potential prejudice envisioned by the
rules. See Fed. R. Evid. 403.


                                - 85 -
              TORRUELLA, Circuit Judge, (Concurring).          To my mind this

is a very close case in which we are second guessing a trial judge

who was faced with a difficult decision and needed to keep a

semblance of order in the court's calendar.                Santiago-Díaz, 456

F.3d at 277 (recognizing the district court's "interest in the

efficient management of its docket").             At the same time, we have

a compelling plaintiff who is left out in the cold, most probably

by actions beyond his personal doing. While I agree with the rules

articulated      herein,    I    point    out   the    practical   difficulties

district court judges confront in their charge to penalize parties

who    have    flouted     important      procedural   rules   like   Rule   26,

particularly where such noncompliance has been perceived by the

court as willful, and caution against dismissing the harm caused

by such noncompliance as an inconvenience.               Finally, I recognize

that    the     district        court's     determinations     regarding     the

unreliability of the expert's evidence under Rule 702 flowed from

the expert's failure to provide detailed conclusions in a final

and complete expert report and advise parties wishing to avoid

exclusion on this basis to make a good faith effort to comply with

disclosure requirements or risk facing serious repercussions.                In

the balance of things, I join my colleagues, but not without

expressing my concern with how our decision will impact future

actions by district court judges in their attempt to administer

their courts with a predictable order.


                                       - 86 -
