                  FOR PUBLICATION
  UNITED STATES COURT OF APPEALS
       FOR THE NINTH CIRCUIT

MARYLOU PRIMIANO; CHARLES              
PRIMIANO,                                   No. 06-15563
              Plaintiffs-Appellants,           D.C. No.
                v.                          CV-03-00373-
                                              JCM/PAL
YAN COOK; STRYKER CORPORATION;
ROBERT J. TAIT M.D.,
                                              ORDER
                         Defendant,         AMENDING
                                           OPINION AND
HOWMEDICA OSTEONICS                          AMENDED
CORPORATION,                                  OPINION
              Defendant-Appellee.
                                       
        Appeal from the United States District Court
                 for the District of Nevada
         James C. Mahan, District Judge, Presiding

        Argued and Submitted February 13, 2008
     Submission Withdrawn and Supplemental Briefing
                Requested March 3, 2008
                Resubmitted July 15, 2009
                San Francisco, California

                   Filed March 10, 2010
                  Amended April 27, 2010

   Before: Dorothy W. Nelson, Andrew J. Kleinfeld, and
          Michael Daly Hawkins, Circuit Judges.

                Opinion by Judge Kleinfeld




                            6281
6284          PRIMIANO v. HOWMEDICA OSTEONICS




                       COUNSEL

Peter C. Wetherall, Las Vegas, Nevada, for plaintiffs-
appellants Marylou and Charles Primiano.

Frederick D. Baker (argued), Wayne A. Wolff, San Francisco,
California; Ralph A. Campillo, Los Angeles, California, for
defendant-appellee Howmedica Osteonics Corporation.


                         ORDER

  The opinion filed on March 10, 2010, and appearing at 598
F.3d 558 (9th Cir. 2010), is amended as follows:

    1.   At page 563, after the sentence “We review
         summary judgment de novo.” and footnote 5,
                 PRIMIANO v. HOWMEDICA OSTEONICS                   6285
           add the sentence “We review rulings on the
           admissibility of expert testimony under Federal
           Rule of Evidence 702 for abuse of discretion.”

      2.   At page 563, at the end of the newly added sen-
           tence, append a new footnote 6 with the follow-
           ing text in the footnote: “Cabrera v. Cordis
           Corp., 134 F.3d 1418, 1420 (9th Cir. 1998).”
           Renumber the remaining footnotes accordingly.

  With this amendment, the panel has unanimously voted to
deny the petition for rehearing. Judge Kleinfeld has voted to
deny the petition for rehearing en banc, and Judges D.W. Nel-
son and Hawkins so recommend. No further petitions for
rehearing will be entertained.

  The full court has been advised of the petition for rehearing
en banc and no judge of the court has requested a vote on the
petition for rehearing en banc. Fed. R. App. P. 35.

  The petition for panel rehearing and the petition for rehear-
ing en banc are DENIED.


                              OPINION

KLEINFELD, Circuit Judge:

 We address admissibility under Daubert1 of medical testi-
mony.

                              I.   Facts

  Marylou Primiano has suffered a miserable ordeal since she
had elbow surgery. The question raised by her litigation2 is
  1
    Daubert v. Merrell Dow Pharm., Inc., 509 U.S. 579 (1993).
  2
    The complaint also names Mr. Primiano as a plaintiff, for his deriva-
tive claim for loss of consortium etc., and names Stryker Corporation as
6286             PRIMIANO v. HOWMEDICA OSTEONICS
whether her ordeal resulted from a defective product, the arti-
ficial elbow Howmedica Osteonics Corporation manufac-
tured. The district court granted summary judgment against
her and dismissed her case, but that result could not have
occurred had her medical expert’s testimony been considered.
His testimony would have established a genuine issue of
material fact, because he thought the plastic bearing between
the metal parts of the artificial elbow wore out so quickly that
it must have been defective. The district court ruled that his
testimony was inadmissible, leaving Primiano with inade-
quate evidence to establish a genuine issue of fact. The ques-
tion before us is whether excluding Primiano’s expert’s
testimony was an abuse of discretion.

   Ms. Primiano, an active 36-year-old woman, fell in her
kitchen and broke her elbow. The injury, serious for anyone,
was especially serious for her, because she has had rheuma-
toid arthritis for years. Unlike osteoarthritis, a degenerative
process of wear and tear on the joints, rheumatoid arthritis is
a chronic inflammatory disease of the connective tissue in the
joints.3 Her physician, Robert J. Tait, M.D., performed sur-
gery April 18, 2000, two days after her fall. He replaced her
elbow joint with a device made by the defendant, How-
medica, consisting of titanium pieces to replace the bone and
polyethylene components to prevent the metal from rubbing
against metal.

   Two thirds of the way through surgery, Dr. Tait discovered
that Howmedica had made a mistake in the packing and ship-
ping, so even though he was replacing Ms. Primiano’s right
elbow, the humeral component (the humerus is the arm bone

owner of Howmedica Osteonics Corporation, Robert J. Tait M.D., the sur-
geon who operated on Ms. Primiano, and Yan Cook, a Howmedica sales
representative. Only the Primianos’ appeal challenging the summary judg-
ment and exclusion of evidence in favor of Howmedica is before us.
  3
    Blakiston’s Gould Medical Dictionary 1353 (3d ed. 1972).
               PRIMIANO v. HOWMEDICA OSTEONICS            6287
running from the elbow to the shoulder) sent to him was
labeled for the left arm. He consulted Howmedica’s represen-
tative (“Did I kill him? No, I didn’t.”) with Ms. Primiano’s
arm open on the table and was told that the components are
symmetrical, identical in every respect except that the locking
pin goes in the opposite side of the left humeral component,
so the component he had could be used. The hole had to be
drilled in Ms. Primiano’s bone from the inside instead of the
outside, but the artificial joint would be equally functional.
Dr. Tait completed the operation, and it appeared to be a suc-
cess.

   But by July, Ms. Primiano’s elbow squeaked, and by
December, Dr. Tait could hear the metal-on-metal contact,
which he confirmed in an x-ray. In February, Dr. Tait per-
formed a second surgery addressing the evident failure of the
implant and risk of metallosis (a destructive immune response
of the body to flecks of metal shaved off by metal-on-metal
contact), replacing the humeral component with a longer one.
He used Howmedica’s left arm humeral component again,
though the long instead of the standard, to avoid having to
redrill the remaining bone. He observed massive metallosis
and “severe polyethylene wear” on the bearing surrounding
the pin. Again, the surgery appeared to go fine. But the next
month, Ms. Primiano was having trouble controlling her arm
and the joint had a “cracking” sound. She obtained a second
opinion from an orthopedic surgeon who concluded that the
components appeared “to be adequately fixed and in good
position.” But in June her problems with the joint had not
gone away, so she consulted a third orthopedic surgeon, who
recommended a third surgery. In July this surgeon replaced
her Howmedica device with one from its competitor, Zimmer.
That surgeon performed a fourth surgery the next April to cor-
rect loosening. A pin backed out of position, so she needed
yet another surgery, her fifth, in September.

  Primiano sued Howmedica, Dr. Tait, and others in state
court for negligence, strict liability, breach of warranty, and
6288             PRIMIANO v. HOWMEDICA OSTEONICS
loss of consortium.4 Howmedica removed the case to federal
court based on diversity. All that is before us now is the prod-
ucts liability case.

   In the summary judgment papers, Howmedica’s experts, an
orthopedic surgeon and a chemist, provided opinions that the
polyethylene was as it should be, and the rapid failure of the
prosthesis and excessive wear on the polyethylene compo-
nents resulted from “malalignment of the prosthesis” along
with increased risk of complication because of Ms. Primi-
ano’s rheumatoid arhtritis and her age. The product literature
distributed to physicians said that the prosthesis would not
restore function to the level expected with normal healthy
bone, and was vulnerable to excessive loading from activity.
Evidently, younger patients such as Ms. Primiano may do
worse because they are more active. The manufacturer’s liter-
ature says “[w]hile the expected life of the total elbow
replacement components is difficult to estimate, it is finite.”

   Primiano’s expert witness, Arnold-Peter Weiss, M.D.,
declared that the polyethylene bushing had worn through in
less than eight months, “not a usual or expected circum-
stance.” Though finite, the typical lifespan of elbow prosthe-
ses “far exceeds” how long this one lasted. Dr. Weiss testified
in his deposition that although wear starts immediately, elbow
prostheses last as long as ten or fifteen years, even twenty,
and the earliest he had seen them wear out was around five
to eight years, varying with the patient’s activity level.
Though misalignment could cause excessive wear, he had
looked at the x-rays and found no significant misalignment.
Nor would ordinary daily activity produce such extraordinar-
ily rapid wear. Nor could he find technically inappropriate use
of the prosthesis by Dr. Tait. His opinion was that the extraor-
dinarily rapid wear was caused by abrasive wear and genera-
tion of debris from movement of the titanium against the
  4
    Primiano’s complaint says that she is not suing Dr. Tait for malprac-
tice, just as an agent of Howmedica in selling the prosthesis.
                 PRIMIANO v. HOWMEDICA OSTEONICS                     6289
polyethylene. And he concluded that the prosthesis failed to
perform in a manner reasonably to be expected by a surgeon
using it, because it failed too early.

   The district court granted defendants’ motion to exclude
Dr. Weiss’s testimony as not meeting the Daubert standard
and granted summary judgment. The court concluded that Dr.
Weiss’s testimony would not be helpful to the jury. The judge
reasoned: “Well, I mean it’s like res ipsa loquitur, the elbow
failed. Now, why did it fail? Maybe it was malpractice,
maybe it was Dr. Tait.” The evidence of rapid wear “doesn’t
make it defective.” “I think [Dr. Tait’s] opinion is weakened
by the fact that he didn’t see the plaintiff. He didn’t examine
her. He didn’t talk to her.” “[T]here’s no peer review . . . no
publication . . . there’s got to be an objective source that he
relies on.” The court rejected plaintiff’s argument, that testi-
mony that the premature failure was not attributable to over-
use, medical malpractice, “her physiology,” or other factors
external to the device, would assist the jury.

                             II.   Analysis

   We review summary judgment de novo.5 We review rulings
on the admissibility of expert testimony under Federal Rule of
Evidence 702 for abuse of discretion.6 The substantive ques-
tion the jury would have to answer, in this diversity case aris-
ing out of state tort law, is established by Nevada law. The
question whether evidence is admissible, though, is governed
by federal law. The Federal Rules of Evidence “govern pro-
ceedings in the courts of the United States.”7 That is generally
true in diversity cases because the Federal Rules of Evidence
are statutes enacted by Congress.8 Though there are excep-
  5
    Carmen v. San Francisco Unified Sch. Dist., 237 F.3d 1026, 1029 (9th
Cir. 2001).
  6
    Cabrera v. Cordis Corp., 134 F.3d 1418, 1420 (9th Cir. 1998).
  7
    Fed. R. Evid. 101.
  8
    Sims v. Great Am. Life Ins. Co., 469 F.3d 870, 878-79 (10th Cir. 2006).
6290             PRIMIANO v. HOWMEDICA OSTEONICS
tions, such as state substantive law in the guise of an evidenti-
ary rule,9 no exception applies here.

   [1] Ms. Primiano’s burden was to establish a defect in the
manufacture of the artificial elbow. In Nevada, “those prod-
ucts are defective which are dangerous because they fail to
perform in the manner reasonably to be expected in light of
their nature and intended function.”10 A plaintiff need not
“produce direct evidence of a specific product defect [or]
negate any alternative causes of the accident.”11 An “unex-
pected, dangerous malfunction” suffices.12

   [2] Federal Rule of Evidence 702 controlled admissibility
of Dr. Weiss’s opinion. That rule establishes several require-
ments for admissibility: (1) the evidence has to “assist the
trier of fact” either “to understand the evidence” or “to deter-
mine a fact in issue”; (2) the witness has to be sufficiently
qualified to render the opinion:

      If scientific, technical, or other specialized knowl-
      edge will assist the trier of fact to understand the evi-
      dence or to determine a fact in issue, a witness
      qualified as an expert by knowledge, skill, experi-
      ence, training, or education, may testify thereto in
      the form of an opinion or otherwise, if (1) the testi-
      mony is based upon sufficient facts or data, (2) the
      testimony is the product of reliable principles and
  9
    See Feldman v. Allstate Ins. Co., 322 F.3d 660, 666 (9th Cir. 2003);
Wray v. Gregory, 61 F.3d 1414, 1417 (9th Cir. 1995) (per curiam).
   10
      Allison v. Merck & Co., 878 P.2d 948, 952 (Nev. 1994) (internal quo-
tation marks omitted); Ginnis v. Mapes Hotel Corp., 470 P.2d 135, 138
(Nev. 1970) (internal quotation marks omitted).
   11
      Stackiewicz v. Nissan Motor Corp., USA, 686 P.2d 925, 927 (Nev.
1984).
   12
      Id. at 928.
                  PRIMIANO v. HOWMEDICA OSTEONICS                     6291
       methods, and (3) the witness has applied the princi-
       ples and methods reliably to the facts of the case.13

Though Daubert is sometimes loosely spoken of as though it
established the court’s “gatekeeping” function, that is not
quite right. Trial courts have always had a gatekeeping func-
tion for opinion evidence. Daubert held that Federal Rule of
Evidence 702 replaces the old Frye14 gatekeeping test, “gen-
eral acceptance in the particular field,” with a different test
which is, in some respects, more open to opinion evidence.15

   [3] The requirement that the opinion testimony “assist the
trier of fact” “goes primarily to relevance.”16 For scientific
opinion, the court must assess the reasoning or methodology,
using as appropriate such criteria as testability, publication in
peer reviewed literature, and general acceptance, but the
inquiry is a flexible one.17 Shaky but admissible evidence is
to be attacked by cross examination, contrary evidence, and
attention to the burden of proof, not exclusion.18 In sum, the
trial court must assure that the expert testimony “both rests on
a reliable foundation and is relevant to the task at hand.”19
Kumho Tire Co. v Carmichael holds that the Daubert frame-
work applies not only to scientific testimony but to all expert
testimony.20 It emphasizes, though, that the “test of reliability
is ‘flexible’ and Daubert’s list of specific factors neither nec-
  13
      Fed. R. Evid. 702.
  14
      Frye v. United States, 293 F. 1013 (D.C. Cir. 1923).
   15
      Daubert, 509 U.S. at 588 (“Nothing in the text of [Rule 702] estab-
lishes ‘general acceptance’ as an absolute prerequisite to admissibility.”);
id. at 589 (“That austere standard, absent from, and incompatible with, the
Federal Rules of Evidence, should not be applied in federal trials.”).
   16
      Id. at 591.
   17
      Id. at 592-4.
   18
      Id. at 596.
   19
      Id. at 597.
   20
      Kumho Tire Co. v. Carmichael, 526 U.S. 137, 147 (1999); see also
White v. Ford Motor Co., 312 F.3d 998, 1007 (9th Cir. 2002).
6292             PRIMIANO v. HOWMEDICA OSTEONICS
essarily nor exclusively applies to all experts or in every case.”21
The “list of factors was meant to be helpful, not definitive,”22
and the trial court has discretion to decide how to test an
expert’s reliability as well as whether the testimony is reliable,23
based on “the particular circumstances of the particular case.”24

   [4] We further interpreted Daubert on remand.25 In that
case, the evidence proffered was scientific epidemiological
evidence, of insufficient reliability for admissibility. We took
pains to point out that the problem was methodology, not the
conclusion to which the evidence would lead. “[T]he test
under Daubert is not the correctness of the expert’s conclu-
sions but the soundness of his methodology.”26 Under
Daubert, the district judge is “a gatekeeper, not a fact finder.”27
When an expert meets the threshold established by Rule 702
as explained in Daubert, the expert may testify and the jury
decides how much weight to give that testimony.

  Testimony by physicians may or may not be scientific evi-
dence like the epidemiologic testimony at issue in Daubert.
The classic medical school texts,28 Cecil29 and Harrison,30
explain that medicine is scientific, but not entirely a science.
  21
      Kumho Tire, 526 U.S. at 141.
  22
      Id. at 151.
   23
      Id. at 152.
   24
      Id. at 150.
   25
      Daubert v. Merrell Dow Pharmaceuticals, Inc., 43 F.3d 1311, 1313
(9th Cir. 1995).
   26
      Id. at 1318.
   27
      United States v. Sandoval-Mendoza, 472 F.3d 645, 654 (9th Cir.
2006).
   28
      Jock Murray, Neurology Texts for Internists, 123 Annals of Internal
Med. 477, 477-79 (1995).
   29
      Cecil Textbook of Medicine 1 (James B. Wyngaarden & Lloyd H.
Smith Jr. eds., 17th ed. 1985).
   30
      Harrison’s Principles of Internal Medicine 3 (Dennis L. Kasper et al.
eds., 16th ed. 2005).
                 PRIMIANO v. HOWMEDICA OSTEONICS                     6293
“[M]edicine is not a science but a learned profession, deeply
rooted in a number of sciences and charged with the obliga-
tion to apply them for man’s benefit.”31 “Evidence-based
medicine” is “the conscientious, explicit and judicious use of
current best evidence in making decisions about the care of
individual patients.”32 “Despite the importance of evidence-
based medicine, much of medical decision-making relies on
judgment—a process that is difficult to quantify or even to
assess qualitatively. Especially when a relevant experience
base is unavailable, physicians must use their knowledge and
experience as a basis for weighing known factors along with
the inevitable uncertainties” to “mak[e] a sound judgment.”33

   When considering the applicability of Daubert criteria to
the particular case before the court, the inquiry must be flexi-
ble. Peer reviewed scientific literature may be unavailable
because the issue may be too particular, new, or of insuffi-
ciently broad interest, to be in the literature.34 Lack of cer-
tainty is not, for a qualified expert, the same thing as
guesswork.35 “Expert opinion testimony is relevant if the
knowledge underlying it has a valid connection to the perti-
nent inquiry. And it is reliable if the knowledge underlying it
has a reliable basis in the knowledge and experience of the
relevant discipline.”36 “[T]he factors identified in Daubert
may or may not be pertinent in assessing reliability, depend-
ing on the nature of the issue, the expert’s particular expertise,
and the subject of his testimony.”37 Reliable expert testimony
  31
      Cecil Textbook of Medicine, supra, at 1.
  32
      Harrison’s Principles of Internal Medicine, supra, at 3.
   33
      Id.
   34
      Clausen v. M/V New Carissa, 339 F.3d 1049, 1056, 1060 (9th Cir.
2003).
   35
      Id. at 1059.
   36
      Sandoval-Mendoza, 472 F.3d at 654 (internal quotation marks and
citation omitted).
   37
      White v. Ford Motor Co., 312 F.3d 998, 1007 (9th Cir. 2002) (internal
quotation marks omitted).
6294             PRIMIANO v. HOWMEDICA OSTEONICS
need only be relevant, and need not establish every element
that the plaintiff must prove, in order to be admissible.38

   [5] We have some guidance in the cases for applying Dau-
bert to physicians’ testimony. “A trial court should admit
medical expert testimony if physicians would accept it as use-
ful and reliable,” but it need not be conclusive because “medi-
cal knowledge is often uncertain.”39 “The human body is
complex, etiology is often uncertain, and ethical concerns
often prevent double-blind studies calculated to establish sta-
tistical proof.”40 Where the foundation is sufficient, the liti-
gant is “entitled to have the jury decide upon [the experts’]
credibility, rather than the judge.”41 We held in United States
v. Smith that even a physician’s assistant was qualified based
on experience to offer his opinion.42

  [6] Other circuits have taken similar approaches focusing
especially on experience. The Sixth Circuit held that a district
court abused its discretion by excluding a physician’s testi-
mony based on extensive, relevant experience even though he
had not cited medical literature supporting his view.43 Like-
wise the Third Circuit pointed out that a doctor’s experience
might be good reason to admit his testimony.44 Thus under our
precedents and those of other circuits, the district court in this
case was pushing against the current, but that alone does not
imply an abuse of discretion.
  38
     See Stilwell v. Smith & Nephew, Inc., 482 F.3d 1187, 1192 (9th Cir.
2007).
  39
     Sandoval-Mendoza, 472 F.3d at 655.
  40
     Id.
  41
     Id. at 656.
  42
     520 F.3d 1097, 1105 (9th Cir. 2008).
  43
     Dickenson v. Cardiac & Thoracic Surgery of E. Tenn., 388 F.3d 976,
982 (6th Cir. 2004).
  44
     Schneider ex rel. Estate of Schneider v. Fried, 320 F.3d 396, 406-07
(3d Cir. 2003).
                 PRIMIANO v. HOWMEDICA OSTEONICS                  6295
   [7] A close look at the foundation for Dr. Weiss’s opinion,
the nature of medical opinion, and the question posed by
Nevada law does. Dr. Weiss is a board certified orthopedic
surgeon and a professor at Brown University School of Medi-
cine in the Division of Hand, Upper Extremity and
Microvascular Surgery, department of Orthopedics. He has
published over a hundred articles in peer-reviewed medical
journals including several specifically on the elbow and at
least one somewhat related to this case, “Capitellocondylar
Total Elbow Replacement: A Long-Term Follow-up Study.”45
He has years of experience implanting various elbow pros-
thetics and has performed five to ten revisions of total elbow
replacements that had been performed by other physicians. He
has examined the various types of prosthetics available, and
has maintained familiarity with the peer-reviewed literature.
He testified that the very short lifespan of Ms. Primiano’s arti-
fical elbow is “outside of my review of the known literature.”
He conceded on cross examination that there was “no pub-
lished peer-reviewed article that [I’m] aware of that states a
strict minimum lifespan of a polyethylene component in a
total elbow system,” but explained that “I wouldn’t expect
any literature, because you don’t see it. It’s hard to write a
paper about something that doesn’t occur. I mean, this is
really bizarre.”

   [8] A court would have to find that Dr. Weiss is “qualified
as an expert by knowledge, skill, experience, training, or educa-
tion”46 to render an opinion on elbow replacements. The dis-
trict court appears to have rejected the opinion based in part
on two elements of Rule 702, whether his opinion would
assist the trier of fact, and whether it was based upon suffi-
cient facts or data.
  45
      Andrew J. Weiland, Arnold-Peter C.Weiss, Robert P. Wills & J. Rus-
sell Moore, Capitellocondylar Total Elbow Replacement: A Long-Term
Follow-up Study, 71 J. of Bone & Joint Surgery, 217, 217-22 (1989).
   46
      Fed. R. Evid. 702.
6296             PRIMIANO v. HOWMEDICA OSTEONICS
   [9] The district court thought Dr. Weiss’s opinion would
not assist the jury because Dr. Weiss could not say why the
plastic part of the artificial elbow failed so quickly. The “will
assist” requirement, under Daubert, “goes primarily to rele-
vance.”47 What is relevant depends on what must be proved,
and that is controlled by Nevada law. Nevada law establishes
that “those products are defective which are dangerous
because they fail to perform in the manner reasonably to be
expected in light of their nature and intended function.”48 In
Nevada, a plaintiff need not “produce direct evidence of a
specific product defect [or] negate any alternative causes of
the accident.”49 An “unexpected, dangerous malfunction” suf-
fices.50 Since Dr. Weiss, with a sufficient basis in education
and experience, testified that the artificial joint “fail[ed] to
perform in the manner reasonably to be expected in light of
[its] nature and intended function,” that was enough to assist
the trier of fact. He did not have to know why it failed.

  [10] The district court’s other concerns, that Dr. Weiss
never saw or talked to Ms. Primiano, and there was no publi-
cation supporting his opinion that the device failed extraordi-
narily early, both might be useful to the jury as impeachment,
but neither furnished an adequate basis for excluding his opin-
ion. What he most needed to see was what was inside her arm,
not outside it, and he did. He saw the x-rays. He also saw the
polyethylene from the implant installed in Primiano’s first
surgery. As for lack of a publication backing his opinion up,
Daubert offers several reasons why an opinion unsupported
by peer-reviewed publication may be admissible,51 and Dr.
  47
      Daubert, 509 U.S. at 591.
  48
      Allison v. Merck & Co., 878 P.2d 948, 952 (Nev. 1994) (internal quo-
tation marks omitted); Ginnis v. Mapes Hotel Corp., 470 P.2d 135, 138
(Nev. 1970) (internal quotation marks omitted).
   49
      Stackiewicz v. Nissan Motor Corp., USA, 686 P.2d 925, 927 (Nev.
1984).
   50
      Id. at 928.
   51
      Daubert, 509 U.S. at 593.
                 PRIMIANO v. HOWMEDICA OSTEONICS                    6297
Weiss furnished another one, that the phenomenon is so
extraordinary that the specialists who publish articles do not
see it in their practices.

   [11] Dr. Weiss’s background and experience, and his
explanation of his opinion, leave room for only one conclu-
sion regarding its admissibility. It had to be admitted. Once
admitted, the opinion precluded summary judgment, because
if the jury accepted it, then the Howmedica prosthesis “fail-
[ed] to perform in the manner reasonably to be expected.”52
His methodology, essentially comparison of what happened
with Ms. Primiano’s artificial elbow with what surgeons who
use artificial elbows ordinarily see, against a background of
peer-reviewed literature, is the ordinary methodology of evi-
dence based medicine: “not a science but a learned profession
deeply rooted in a number of sciences,”53 “the conscientious,
explicit and judicious use of current best evidence in making
decisions about the care of individual patients”54 and “rel[y-
ing] on judgment—a process that is difficult to quantify or
even to assess qualitatively. Especially when a relevant expe-
rience base is unavailable, physicians must use their knowl-
edge and experience as a basis for weighing known factors
along with the inevitable uncertainties” to “mak[e] a sound judg-
ment.”55

   [12] The jury may reject Dr. Weiss’s opinion. It may con-
clude that Ms. Primiano’s level of activity, or error by Dr.
Tait in performing the surgery, caused the failure. Or it may
conclude that the negligence that matters was in the packing
and shipping department of Howmedica, when they sent the
wrong pieces to the hospital. But those possibilities bear on
  52
     Allison, 878 P.2d at 952.
  53
     Cecil Textbook of Medicine 1 (James B. Wyngaarden & Lloyd H.
Smith Jr. eds., 17th ed. 1985).
  54
     Harrison’s Principles of Internal Medicine 3 (Dennis L. Kasper et al.
eds., 16th ed. 2005).
  55
     Id.
6298            PRIMIANO v. HOWMEDICA OSTEONICS
the merits of Ms. Primiano’s claim, not the admissibility of
Dr. Weiss’s opinion. Given that the judge is “a gatekeeper,
not a fact finder,”56 the gate could not be closed to this rele-
vant opinion offered with sufficient foundation by one quali-
fied to give it.

  REVERSED.




  56
    Sandoval-Mendoza, 472 F.3d at 654.
