                     FOR PUBLICATION

    UNITED STATES COURT OF APPEALS
         FOR THE NINTH CIRCUIT


 UNITED STATES OF AMERICA ,                       No. 12-30078
                 Plaintiff-Appellee,
                                                    D.C. No.
                     v.                          3:09-cr-05810-
                                                     RBL-1
 BILLY MIRANDA FLORES,
             Defendant-Appellant.
                                                    OPINION


        Appeal from the United States District Court
          for the Western District of Washington
        Ronald B. Leighton, District Judge, Presiding

                    Argued and Submitted
              June 7, 2013—Seattle, Washington

                      Filed August 2, 2013

      Before: Arthur L. Alarcón, Ronald Lee Gilman,*
            and Sandra S. Ikuta, Circuit Judges.

                   Opinion by Judge Alarcón




 *
   The Honorable Ronald Lee Gilman, Senior Circuit Judge for the U.S.
Court of Appeals for the Sixth Circuit, sitting by designation.
2                   UNITED STATES V . FLORES

                           SUMMARY**


                           Criminal Law

     Vacating a sentence and remanding for resentencing, the
panel held that any error in estimating the quantity of drugs
attributable to a conspiracy to distribute oxycodone was
harmless, but that the district court erred in imposing without
an adequate factual basis an enhancement under U.S.S.G.
§ 3B1.4 for use of a person less than eighteen years of age.


                             COUNSEL

Jonathan I. Edelstein, Law Office of Alan Ellis, New York,
New York, for Defendant-Appellant.

Teal Luthy Miller, Assistant United States Attorney (argued),
and Jenny A. Durkan, United States Attorney, Office of the
United States Attorney, Seattle, Washington, for Plaintiff-
Appellee.


                             OPINION

ALARCÓN, Circuit Judge:

   Billy Miranda Flores (“Flores”) was convicted on one
count of conspiracy to distribute oxycodone, three counts of


  **
     This summary constitutes no part of the opinion of the court. It has
been prepared by court staff for the convenience of the reader.
                    UNITED STATES V . FLORES                             3

distribution of oxycodone, one count of possession with
intent to distribute methadone, and one count of possession of
a firearm in furtherance of the methadone charge. The
district court sentenced Flores to 240 months imprisonment—
180 months for the drug charges and 60 months for the
firearm charge, imposed consecutively. Flores appeals his
conviction of the gun charge and his sentence. We have
jurisdiction under 28 U.S.C. § 1291 and 18 U.S.C. § 3742(a).
In this opinion, we consider only Flores’s challenges to his
sentence. In a concurrently filed memorandum disposition,
we address Flores’s separate challenges to his conviction.
See United States v. Flores, __Fed. App’x __ (9th Cir. 2013).

     Flores argues that in imposing the sentence, the district
court erred in estimating the quantity of drugs attributable to
the conspiracy under the United States Sentencing
Guidelines, and in applying a two-level sentencing
enhancement under United States Sentencing Guidelines
Manual § 3B1.4 for use of a person less than eighteen years
of age in the conspiracy to distribute oxycodone.1 We
conclude that any error in estimating the quantity of drugs
attributable to the conspiracy was harmless, but that the
district court erred in imposing the sentencing enhancement
without an adequate factual basis. We accordingly vacate
Flores’s sentence and remand for resentencing.




   1
      Flores also contends that the district court erred in imposing a
substantively unreasonable sentence. Given our holding, we do not
address the substantive reasonableness of the sentence. See United States
v. Kilby, 443 F.3d 1135, 1140 (9th Cir. 2006) (holding that if a reviewing
court finds a material error in the Guidelines calculation, it “will remand
for resentencing, without reaching the question of whether the sentence as
a whole is reasonable”).
4                   UNITED STATES V . FLORES

                                    I

    This case arises out of a joint Federal Bureau of
Investigation (“FBI”) and Drug Enforcement Agency
(“DEA”) investigation into oxycodone2 and methadone
trafficking on Puyallup tribal land in Washington State.
Sometime before the summer of 2008,3 law enforcement
began investigating Flores and the smoke shop he managed,
the Indian Smoke Shop (the “Shop”), which is located on the
Puyallup reservation. The investigation employed physical
and electronic surveillance, telephone toll records, trap and
trace devices on target telephones, a pole camera installed


    2
   At trial, the attorneys and witnesses (and consequently the presentence
report, transcripts, parties’ papers, and orders) used the terms
“oxycodone” and “OxyContin” interchangeably. “Oxycodone” is a
generic opioid pain reliever found in a number of prescription medications
and is the sole active ingredient in OxyContin, a widely used prescription
narcotic manufactured by Purdue Pharma L.P.              See U.S. Gov’t
Accountability Office, GAO-04-110, Prescription Drugs: OxyContin
Abuse and Diversion Efforts to Address the Problem 1–2, 8–9, 11 (2003).
Oxycodone is classified as a Schedule II controlled substance “because it
has high potential for abuse and may lead to severe psychological or
physical dependence.” Id. at 2 & n.6. W e use the terms “oxycodone” and
“OxyContin” interchangeably in our opinion to reflect their usage in the
record.

        3
      T he record is unclear as to the start date of the two separate
investigations that eventually merged into the joint investigation. DEA
Investigator Errin Jewell testified that he was informed of an ongoing
DEA investigation in June 2008. He learned that the investigation “had
potential to be very complex, a bigger investigation involving the Indian
Smoke Shop . . . and [he] was assigned as the lead investigator on the
investigation.” FBI Special Agent Donald Treat testified that the FBI’s
South Sound Gang Task Force was conducting an investigation before it
joined with the DEA’s Tacoma Narcotics Enforcement Team task force
around early to mid-2008.
                 UNITED STATES V . FLORES                    5

outside the Shop, confidential informants, and controlled
purchases of pills by undercover agents and cooperators. The
investigation ended on October 20, 2009, when law
enforcement officers arrested Flores and his codefendants:
Bill C. Flores (“Flores Senior”), Flores’s father; Tiny Bean-
Flores (“Tiny Bean”), Flores’s half-brother and an employee
of the Shop; Danny Lee Sherwood (“Sherwood”), Flores’s
relative and an employee of the Shop; Timothy Morehead, a
purchaser; and Shelbie Ingham (“Ingham”), Flores’s
girlfriend at the time.

    The same day, law enforcement officers searched the
Shop and Flores’s house pursuant to search warrants and
uncovered evidence of a drug conspiracy.            Officers
discovered cash and two bottles of methadone, and a loaded
Smith and Wesson, Model 29, .44 Magnum revolver in an
unlocked drawer approximately one foot from a safe where
the officers recovered the bottles of methadone. The search
of Flores’s house yielded approximately seven empty
prescription bottles—five of which were for oxycodone and
all of which were prescribed to persons other than Flores.

    Based on this evidence, Flores and his codefendants were
indicted and charged in 34 counts. Flores was indicted with
one count of conspiring to distribute oxycodone and
methadone in violation of 21 U.S.C. §§ 841(a)(1), (b)(1)(C),
and 846, five counts of substantive distribution of oxycodone
in violation of 21 U.S.C. §§ 841(a)(1), (b)(1)(C), one count of
possession with intent to distribute methadone in violation of
18 U.S.C. § 2 and 21 U.S.C. §§ 841(a)(1), (b)(1)(C), and one
count of possession of a firearm in furtherance of the
methadone count in violation of 18 U.S.C. §§ 2, 924(c)(1)(A).
Tiny Bean, Ingham, Morehead, and Flores Senior entered into
plea agreements. Flores and Sherwood proceeded to trial. A
6                       UNITED STATES V . FLORES

jury found Flores guilty of conspiring to distribute
oxycodone, distributing oxycodone, possessing methadone
with intent to distribute it, and possessing a gun in
furtherance of the methadone charge.4

    In calculating Flores’s base offense level, Flores’s
Presentence Report (“PSR”) adopted a start date for the
conspiracy of October 2007 and estimated that the conspiracy
sold 1,000 to 1,500 80-milligram pills of oxycodone per day.
PSR recommended a two-level upward adjustment for use of
Shelbie Ingham, a person under eighteen years of age, in the
conspiracy. Flores filed objections to the PSR in his
sentencing memorandum. Specifically, he asserted that (1)
the PSR’s estimate of the quantity of drugs attributable to the
conspiracy was too high, and (2) there was no evidence to
support the enhancement for use of a minor.

    At Flores’s sentencing hearing, Sherwood testified that he
joined the conspiracy in May 2009 and eventually sold up to
1,000 pills daily for Flores. The district court concluded that
a “conservative estimate” placed the start date of the
conspiracy in December 2007, based on an unindicted
coconspirator’s trial testimony. The court noted that it had
heard “a lot of testimony about the volume of the sales” and
concluded that 500 80-milligram pills of oxycodone was a
“conservative[]” estimate of the amount of pills that moved
through the conspiracy daily between December 2007 and
October 2009.

    The district court explained that the marijuana equivalent
for that amount of oxycodone was “well over” 30,000
kilograms of marijuana, which is the smallest quantity that

    4
        The jury acquitted Flores of conspiring to distribute methadone.
                     UNITED STATES V . FLORES                             7

triggers the highest base offense level, 38, in the Drug
Quantity Table. See U.S. Sentencing Guidelines Manual
§ 2D1.1 (Drug Quantity Table).5 The district court therefore
concluded that Flores’s base offense level was 38. The
district court imposed a four-level enhancement for Flores’s
leadership role in the conspiracy and a two-level
enhancement for use of a minor. However, the district court
did not make an express finding as to Ingham’s age at the
time of her involvement in the conspiracy. The court
imposed a two-level downward adjustment for Flores’s
acceptance of responsibility, resulting in a total offense level
of 42. The court correctly stated that the Guidelines range for
an offense level 42 and criminal history category I is 360
months to life.



  5
    Federal sentencing courts use the Drug Quantity Table in § 2D1.1 of
the Federal Sentencing Guidelines to calculate the base offense level for
offenses involving controlled substances. W here, as here, an offense
involves a drug that is not listed in the Drug Quantity Table, sentencing
courts use the Drug Equivalency Tables in § 2D1.1 to covert the drug into
an equivalent amount of marijuana. See U.S. Sentencing Guidelines
Manual § 2D1.1 cmt. n.8(A) (2012). The Drug Equivalency Tables
provide that one gram of actual oxycodone equals 6,700 grams of
marijuana.

     Here, using the district court’s start date of the conspiracy, December
2007, means the conspiracy lasted 689 days. M ultiplying 689 days by the
district court’s estimate of 500 pills per day results in a total of 344,500
80-milligram pills. Multiplying this amount by the 80 milligrams in each
pill results in 27,560,000 milligrams, or 27,560 grams, of oxycodone.
Finally, after converting oxycodone to its marijuana equivalent using the
1:6,700 gram ratio in the Drug Equivalency Tables, the district court’s
adopted start date and daily quantity yield a total of 184,652 kilograms of
marijuana. Pursuant to the Drug Quantity Table, any amount of marijuana
over 30,000 kilograms results in the highest base offense level, which is
38.
8                 UNITED STATES V . FLORES

       The district court did not rely solely on the Guidelines
range in determining its ultimate sentence. Noting that the
Federal Sentencing Guidelines are advisory, the court
explained that the “big issue is not so much the math, the
calculations; it’s the qualitative issues that are so distressing
. . . . Maybe Shelbie Ingham was broken before Billy Miranda
Flores got his claws in her, but she is definitely broken now.”
The court also stated:

        There is a lot of human wreckage in this case,
        and we don’t even know how pervasive the
        reach of this conspiracy went, not those who
        agreed to perform sales, but just the reach of
        the conspiracy, the people who bought, the
        people who sold and more—most importantly,
        those people who used.

The district court further opined that the “biggest challenge
for any judge who sentences someone” is determining the
difference between a criminal “who is ignorant or dumb or
stupid . . . [and] in need of a break” and those who “are really
bad.” “I put Billy Miranda Flores among that [latter] group”
because Flores “preys on people,” “has no empathy for other
people,” and “has harmed so many people.” Based on “the
seriousness of the offense, the personal characteristics, the
need to promote the rule of law and to deter people from
committing the same offenses that he has committed,” the
court imposed a total of 240 months of imprisonment—180
months of imprisonment on the drug charges, half of the low-
end of the advisory Guidelines range, and 60 months on the
firearm offense, which is the mandatory minimum sentence
for a violation of 18 U.S.C. § 924(c).
                 UNITED STATES V . FLORES                      9

    Flores raises two issues on appeal relating to the
calculation of his Federal Sentencing Guidelines range. First,
Flores argues the district court erred in calculating the amount
of drugs attributable to the conspiracy by employing a
method that was not sufficiently reliable and by failing to “err
on the side of caution” as required by Ninth Circuit
precedent. Second, Flores asserts that the two-level
enhancement for use of a person under eighteen years of age
in the conspiracy was not supported by the record.

                               II

     We first address Flores’s challenge to the district court’s
calculation of the drug quantity attributable to the conspiracy.
“When we review a sentence, the first step is to determine if
the district court made a material error in its Guidelines
calculation that serves as the starting point for its sentencing
decision.” Kilby, 443 F.3d at 1140 (citing United States v.
Cantrell, 433 F.3d 1269, 1280 (9th Cir. 2006)). If we find a
material error in the Guidelines calculation, “we will remand
for resentencing, without reaching the question of whether the
sentence as a whole is reasonable.” Kilby, 443 F.3d at 1140
(citation omitted). If the district court did not err in applying
the Guidelines, we then consider challenges to the
reasonableness of the overall sentence in light of the factors
specified in 18 U.S.C. § 3553(a). Id.

    This Court reviews de novo “whether a district court’s
method of approximation of the relevant drug quantity
conforms to the Guidelines.” Id. (citing United States v.
Rosacker, 314 F.3d 422, 425 (9th Cir. 2002)). After United
States v. Booker, 543 U.S. 220 (2005), we review a district
court’s sentencing decisions, including its factual findings,
for abuse of discretion. Gall v. United States, 552 U.S. 38, 51
10               UNITED STATES V . FLORES

(2007); United States v. Carty, 520 F.3d 984, 993 (9th Cir.
2008) (en banc). Under the law of this Circuit, a district court
is required to resolve factual disputes regarding drug quantity
by applying the preponderance of the evidence standard.
Kilby, 443 F.3d at 1140; see Rosacker, 314 F.3d at 429–30
(holding the clear and convincing standard that applies to
cases where a sentencing factor has an extremely
disproportionate effect on the sentence does not apply to drug
quantity approximations).

    Several different witnesses testified about the quantity of
drugs sold by the conspiracy. Most of the evidence at trial
relating to the quantity of drugs over the life of the conspiracy
came from codefendant Tiny Bean, unindicted coconspirators
Leslie Larson (“Larson”) and Dennis Hilsendeger
(“Hilsendeger”), law enforcement officers who surveilled the
Shop, and an undercover officer who made three controlled
buys from Flores. In addition, Sherwood testified at Flores’s
sentencing hearing regarding the amount of drugs Sherwood
sold for the conspiracy.

    A short summary of the coconspirators’ testimony
relating to the quantity of drugs they individually sold for
Flores is useful here. Larson testified she began selling drugs
for Flores in approximately February of either 2007 or 2008,
while the other coconspirators testified they began selling for
Flores as early as January 2008 and as late as mid-2009.
Larson testified that she sold drugs for Flores for
approximately one year until her arrest on April 16, 2009.
Over this one-year period, Larson testified she started selling
a few pills per day and eventually increased to selling
hundreds of pills per day. Hilsendeger testified he sold pills
for Flores for approximately six months, starting with a few
pills a day in January or February 2008 and eventually
                    UNITED STATES V . FLORES                           11

increasing to 50 or 100 pills per day. Tiny Bean testified he
sold pills for Flores when Flores was out of town and sold
approximately 100 to 300 pills over ten or twelve weekends,
or 50 to 100 pills per day, but once sold around 1,000 pills
over one weekend. Sherwood testified he sold pills for Flores
for around six months starting in May 2009, though he
believed that Flores began selling pills in 2007. Sherwood
testified he sold hundreds of pills per day when he started and
eventually increased to 1,000 pills per day. Sherwood
estimated that eight to ten other persons sold drugs for Flores.
The witnesses testified they primarily sold 80-milligram pills.

    The PSR estimated that the conspiracy sold 1,000 to
1,500 80-milligram pills per day from October 2007 through
October 2009. The district court adopted December 2007 as
the start date of the conspiracy and used 500 80-milligram
pills as a “conservative” estimate of the volume of oxycodone
pills moved by the conspiracy per day.

    The accuracy of this estimate is important because the
applicable Guidelines range for a drug trafficking crime
depends upon the weight of drugs involved. United States v.
Culps, 300 F.3d 1069, 1076 (9th Cir. 2002); U.S. Sentencing
Guidelines Manual § 2D1.1(a), (c) (2004). Where, as here,
none6 of the total drugs allegedly sold by the conspiracy were
seized, the district court may approximate the weight of the
drugs. Kilby, 443 F.3d at 1141 (citing United States v.
August, 86 F.3d 151, 154 (9th Cir. 1996); U.S. Sentencing



  6
     Although 332 methadone pills were seized from Flores’s office, the
jury acquitted Flores of conspiring to distribute methadone. Thus, those
pills are not included in the quantity drugs attributable to the conspiracy
to distribute oxycodone.
12                   UNITED STATES V . FLORES

Guidelines Manual § 2D1.1, cmt. n.5).7 Because a sentence
will vary greatly based on these approximations, courts must
exercise caution in selecting the method of calculation and
meet the following criteria: (1) “the government is required
to prove the approximate quantity by a preponderance of the
evidence . . . [which means that] [t]he district court must
conclude that the defendant is more likely than not actually
responsible for a quantity greater than or equal to the quantity
for which the defendant is being held responsible”; (2) “the
information which supports an approximation must possess
sufficient indicia of reliability to support its probable
accuracy”; and (3) because the defendant’s “sentence depends
in large part upon the amount of drugs . . . and approximation
is by definition imprecise, the district court must err on the
side of caution” in approximating the drug quantity. Culps,
300 F.3d at 1076 (emphasis added) (internal quotation marks
and citation omitted); United States v. Walton, 908 F.2d 1298,
1302 (9th Cir. 1990) (“[W]hen choosing between a number
of plausible estimates of drug quantity, none of which is more
likely than not the correct quantity, a court must err on the
side of caution.”).

    This Court previously has approved the “multiplier
method” employed by the district court, which estimates the
total quantity of drugs sold by “selecting a time period over


 7
     The comment provides:

          In making this determination, the court may consider,
          for example, the price generally obtained for the
          controlled substance, financial or other records, similar
          transactions in controlled substances by the defendant,
          and the size or capability of any laboratory involved.

U.S. Sentencing Guidelines Manual § 2D1.1, cmt. n.5.
                  UNITED STATES V . FLORES                     13

which it is more likely than not that the defendant was
dealing in that quantity and multiplying these two factors
together.” Culps, 300 F.3d at 1077 (holding the multiplier
method is an acceptable means of calculating drug quantity
“[p]rovided that the approximation has a reliable evidentiary
basis and that the court proceeds with caution”). Applying
the multiplier method in this case required the district court
to determine three data points: the length of the conspiracy,
the number of pills sold by the conspiracy per day, and the
tablet strength of the pills. We examine the district court’s
findings as to each data point.

    We begin with the easiest data point first: the tablet
strength. Several witnesses involved in the conspiracy
testified they primarily sold 80-milligram pills, and no
witness contradicted this testimony. In addition, Flores sold
70 to 100 80-milligram pills to an undercover agent during
three controlled buys. Consequently, the district court’s
adoption of an average tablet strength of 80-milligrams was
not an abuse of discretion.

     The next data point we consider is the quantity of drugs
attributable to the conspiracy per day. The district court
discounted its baseline estimate of 1,000 pills per day by fifty
percent to satisfy its duty to account for a margin of error.
See id. at 1081 (stating fifty percent reduction of baseline
estimate ordinarily will “satisfy the district court’s duty to err
on the side of caution[,]” assuming the amount being
discounted is founded on reliable evidence); cf. August,
86 F.3d at 154 (permitting the method of calculating “lab
capacity” as an approximation of the relevant amount of
drugs that “could have been produced during the course of
the offense”) (emphasis in original). There was scant
evidence at trial relating to the quantity of drugs sold during
14                  UNITED STATES V . FLORES

the earliest months of the district court’s selected time frame
for the conspiracy.8 There was, however, coconspirator
testimony attributing hundreds of pills per day to the
conspiracy from January 2008 through mid-2008 and then
even higher amounts for the months thereafter. The district
court’s estimate of 500 80-milligram pills per day is
supported by a preponderance of the evidence. Thus, the
district court did not abuse its discretion in estimating the
daily drug quantity attributable to the conspiracy.

    Finally, as to the last data point, the length of the
conspiracy, the district court adopted a start date of December
2007. In selecting that date, the district court stated that it
relied on Larson’s testimony. Larson testified that she
purchased oxycodone from Flores for approximately one
year. At the sentencing hearing, the district court stated that
Larson was arrested in August 2008 and that nine or twelve
months before her arrest would place her entrance into the
conspiracy in “October to December 2007.” Later, the court
stated:

         I believe the conspiracy began on or before
         December 2007. Leslie Larson was arrested
         in August [2008]. She was buying up to 300
         pills per day from Billy Flores for 9 to 12


  8
    Only Larson both placed the date of the conspiracy during that time
period and provided information regarding the quantity of pills she sold
during that time. However, Larson stated she began selling pills in either
February 2007 or February 2008. Sherwood is the only other witness who
testified that the conspiracy began in 2007 (though he was not involved at
that time), but he provided no information regarding the amount of drugs
sold during that period. Several witnesses— Larson, Hilsendeger, and
Sherwood— testified they began selling small amounts of pills for Flores
in 2008 or 2009 and eventually increased their sales dramatically.
                 UNITED STATES V . FLORES                  15

       months at that time. Taking the conservative
       estimate would put that in December 2007.

Thus, to arrive at December 2007 as the start date of the
conspiracy, the district court relied on August 2008 as the
date Larson was arrested and then counted backwards nine
months to reach December 2007.

    The evidence at trial, however, does not support a finding
that Larson was arrested in August 2008. Although Larson
testified that law enforcement searched her apartment at some
unspecified date before her arrest, she stated that her “first
arrest” for selling drugs for Flores was not until April 16,
2009. Larson testified that she was arrested again in
November 2009 (the day after Thanksgiving) for not
appearing in court and then served six months of
imprisonment.

    A district court may rely on a coconspirator’s testimony
where, as here, the testimony is given under oath and is
subject to cross-examination. See, e.g., United States v.
Alvarez, 358 F.3d 1194, 1213 (9th Cir. 2004) (holding three
coconspirators’ estimates of drug weight were sufficiently
reliable where they testified under oath and were subject to
cross-examination); United States v. Melchor-Zaragoza,
351 F.3d 925, 929 (9th Cir. 2003) (holding that a
coconspirator’s testimony that twenty-three aliens were taken
hostage by conspirators was sufficient to support court’s
finding, for sentencing purposes, that twenty-three victims
were involved in the conspiracy to commit hostage taking).
However, there is no evidence in the record to support the
district court’s finding that Larson was arrested in August
2008. Consequently, the district court’s December 2007 start
date of the conspiracy, which was based on the purported
16               UNITED STATES V . FLORES

August 2008 arrest, also is not supported by Larson’s
testimony.

    Sherwood was the only other witness who testified that
Flores was selling drugs in 2007. In response to Government
questioning at the sentencing hearing, Sherwood confirmed
that he was not “aware of any time period from [an
unspecified date in 2007] until [Sherwood’s] arrest in 2009
when Billy Flores wasn’t selling pills.” The remaining
witnesses testified to entering the conspiracy in 2008 or 2009.
Of those witnesses, Hilsendeger testified to the earliest
involvement—in January or February of 2008.

    The PSR placed the start date of the conspiracy in
October 2007, two months before the date the district court
adopted. A “court may adopt the factual findings of the
presentence report . . . [but] may not . . . adopt conclusory
statements unsupported by facts or the Guidelines.” United
States v. Navarro, 979 F.2d 786, 789 (9th Cir. 1992); see
United States v. Garcia-Sanchez, 189 F.3d 1143, 1149 (9th
Cir. 1999) (reversing estimate of conspiracy’s weekly drug
sales where government offered only agent’s unexplained
conclusions that did not contain “sufficient indicia of
reliability to support its probable accuracy” (citation
omitted)). Sherwood’s testimony gives the district court’s
determination that the conspiracy started in December 2007
some slight support. But even if the district court erred in
determining the start date, any error in the district court’s
calculation of the length of the conspiracy is harmless beyond
a reasonable doubt. See Alvarez, 358 F.3d at 1213 (holding
any error was harmless where, in order to lower defendant’s
base offense level, the quantity of drugs would have to drop
below an amount that was not supported by the record).
Selecting a start date, based on Hilsendeger’s testimony, of
                 UNITED STATES V . FLORES                  17

January or February 2008 would result in the same base
offense level, as would adopting Flores’s suggested start date
of September 25, 2008. Indeed, even if the length of the
conspiracy were cut in half, the number of pills sold by the
conspiracy daily would have to drop below 170 80-milligram
pills to lower Flores’s base offense level from 38 to 36. Such
a drop is not supported by the record. Thus, a recalculation
of the Guidelines range by the district court based on a
shorter conspiracy length would not lead to a lesser base
offense level. See id. Consequently, any error by the district
court in calculating the quantity of drugs attributable to the
conspiracy is harmless. See Cantrell, 433 F.3d at 1280;
United States v. Scheele, 231 F.3d 492, 499–500 & n.4 (9th
Cir. 2000).

                             III

    Flores next argues that the district court committed
procedural error in imposing a sentencing enhancement for
use of a person under eighteen years of age in the conspiracy.
Flores maintains there was no evidence in the record as to
Shelbie Ingham’s age at the time she entered the conspiracy.
We agree that the district court abused its discretion in
imposing this sentencing enhancement because the court
failed to set forth its reasons for finding that Ingham was a
minor when she participated in the conspiracy.

                              A

    At trial, Flores testified that he met Shelbie Ingham
around November or December 2008. After meeting Ingham,
Flores said he did not see her for four to six months because
Ingham made his girlfriend at the time, Larson, “very upset,”
and Larson told Flores not to talk to Ingham. Flores started
18               UNITED STATES V . FLORES

talking to Ingham again after she brought her car to the Shop
for repairs. In his testimony, Flores admitted to selling
Ingham oxycodone in the summer of 2009, when she was
around eighteen years old. Tiny Bean testified he sold drugs
to Ingham sometime before he stopped selling drugs for
Flores in June 2009. In August or September 2009, law
enforcement made a controlled buy from Ingham. Flores
admitted to being intimate with Ingham “at the end,” before
he was arrested on October 20, 2009. Flores estimated that
Ingham turned eighteen about four months before she was
arrested, placing her birthday around June 2009.

    The PSR recommended a two-level upward adjustment
for Flores’s use of “a person . . . less than 18 years of age to
commit the offense.” Flores’s PSR referred to Ingham as a
minor and stated that Ingham was addicted to pills and
became a member of the conspiracy to support her addiction.
Flores objected to the PSR’s characterization of Ingham as a
minor at the time she was involved in the conspiracy, arguing
there was no evidence to support that finding. The probation
office declined to amend the PSR, however, stating that its
information of Ingham’s age was based on the following
language from Ingham’s PSR: “Flores provided a residence
and vehicle for Ingham, who was the youngest member of the
conspiracy, and barely seventeen when she became involved
with Oxycontin and met Flores.”

    In its Sentencing Memorandum, the Government did not
request an enhancement for use of a minor pursuant to
§ 3B1.4, nor did it allege facts demonstrating that Ingham
was a minor. It indicated at the sentencing hearing, however,
that it did not “disagree with the Court that there is evidence
there to find it.”
                 UNITED STATES V . FLORES                    19

    The district court failed to point to any evidence in the
record establishing Ingham’s age at the time of her
involvement in the conspiracy. During the sentencing
hearing, defense counsel argued that the enhancement for use
of a minor in the conspiracy should not apply, and the district
court responded: “I can tell you that . . . there’s a two-level
increase on use of a person under [eighteen]. I know that
from—I know [Ingham], and I know the circumstances.”
After listening to Sherwood’s testimony, the argument of
counsel, and Flores’s allocution, the district court imposed
the two-level enhancement for using a person less than
eighteen years of age in the conspiracy. The district court’s
vague statement about knowing Ingham and the
circumstances did not set forth any facts to justify the
imposition of the enhancement.

                               B

    Post-Booker, we review a district court’s sentencing
decisions, including its factual findings, for abuse of
discretion. Gall, 552 U.S. at 49; Carty, 520 F.3d at 993. A
two-level sentencing enhancement may be imposed under
United States Sentencing Guidelines Manual § 3B1.4 “[i]f the
defendant used or attempted to use a person less than eighteen
years of age to commit the offense or assist in avoiding
detection of, or apprehension for, the offense,” meaning that
the defendant directed, commanded, encouraged, intimidated,
counseled, trained, procured, recruited, or solicited the minor.
U.S.S.G. § 3B1.4 cmt. n.1 (2001). The district court may
impose the § 3B1.4 enhancement only if a preponderance of
the evidence demonstrates that the defendant “acted
affirmatively to involve the minor in the crime.” United
States v. Jimenez, 300 F.3d 1166, 1069 (9th Cir. 2002)
(quotation marks and citation omitted).
20               UNITED STATES V . FLORES

    Because the district court did not provide a factual basis
for imposing the enhancement for use of a minor in the
conspiracy, we must determine whether an “adequate
explanation” can be “inferred from the PSR or the record as
a whole.” Carty, 520 F.3d at 992; see also United States v.
Jordan, 291 F.3d 1091, 1099 (9th Cir. 2002) (“A merely
speculative logic cannot displace the need for evidence on
[the question of whether a defendant’s conduct justified a
leadership-role enhancement], which cannot be decided by
assumption or inference not based on fact.”). Here, neither
the PSR nor the record supply an adequate explanation for
this enhancement.

    At trial, Flores admitted that he conspired with Ingham to
sell oxycodone but denied that she was a minor when she
began selling for him. There was no other evidence at trial
regarding Ingham’s age when she entered the conspiracy.
The PSR refers to Ingham as a minor and states that Ingham
was addicted to pills and became a member of the distribution
conspiracy to support her own addiction. The Addendum to
the PSR, created in response to Flores’s objection, copied
language from Ingham’s PSR, which states that Flores
provided Ingham a residence and a vehicle and that she was
“barely seventeen when she became involved with Oxycontin
and met Flores.” This factual assertion does not support a
finding that she became a member of the conspiracy before
she reached the age of eighteen.

    The first question, then, is whether these statements in the
PSR may serve as a proper basis for the district court’s
decision. This Court has held that a district court may rely on
an unchallenged PSR at sentencing to find by a
preponderance of the evidence that the facts underlying a
sentencing enhancement have been established. See United
                 UNITED STATES V . FLORES                    21

States v. Charlesworth, 217 F.3d 1155, 1160–61 (9th Cir.
2000) (where defendant failed to challenge accuracy of
information in presentence report and failed to offer any
evidence to contradict presentence report, sentencing court
did not err in relying on presentence report in denying
defendant’s request for sentence reduction); United States v.
Marin-Cuevas, 147 F.3d 889, 895 (9th Cir. 1998). However,
the district court “may not . . . adopt conclusory statements
unsupported by the facts or the Guidelines.” Culps, 300 F.3d
at 1078 (citations omitted) (holding a conclusory statement in
a PSR regarding the average transaction size in a drug
conspiracy could not sustain district court’s adoption of that
size where “the factual underpinning for that assertion, if any,
was not explained”).

    The record does not provide an explanation of the factual
underpinning for the assertion in the PSR that Ingham was a
minor when she participated in the conspiracy. Flores
objected to the PSR and argued there was no evidence of
Ingham’s age when she joined the conspiracy. He did not
submit evidence regarding her age when she joined the
conspiracy. Flores’s failure to submit such evidence is not
dispositive. We previously have held that “when a defendant
raises objections to the PSR, the district court is obligated to
resolve the factual dispute, and the government bears the
burden of proof . . . . The court may not simply rely on the
factual statements in the PSR.” United States v. Showalter,
569 F.3d 1150, 1160 (9th Cir. 2009) (emphasis added) (citing
United States v. Ameline, 409 F.3d 1073, 1085–86 (9th Cir.
2005) (en banc); Fed. R. Crim. P. 32(i)(3)(B) (requiring court
to rule on disputed matters at sentencing)). Thus, Flores’s
failure to provide additional evidentiary support for his
contention did not alter the district court’s obligation to
22                  UNITED STATES V . FLORES

resolve this factual dispute nor excuse the Government’s
failure to meet its burden of proof.

    The evidence in the record similarly does not provide the
necessary factual underpinning for imposing the
enhancement. The only evidence in the record of Ingham’s
age came from Flores, who estimated that she turned eighteen
in June 2009.9 None of the events the Government relies on
to connect Ingham to the conspiracy took place before June
2009. The Government identifies activities that took place in
the “summer,” but “summer” falls both before and after
Ingham’s birthday. Nor does evidence that Ingham was
“close” with Flores or in a relationship with him before she
turned eighteen establish by a preponderance of the evidence
that she was involved in the conspiracy at that time. The only
evidence affirmatively linking Ingham to the conspiracy—as

  9
    In its answering brief, the Government states that Ingham’s birthday
“was in July 2009.” The Government determined Ingham’s birthday by
looking at Ingham’s Presentence Report, “to which the district court had
access.” Ingham’s PSR is not properly before this Court, however,
because it was not in the record below. The Government alternatively
argues that the PSR Addendum provided Flores access to the relevant
information from Ingham’s PSR regarding her age. This is incorrect,
however, because the Addendum did not include Ingham’s birthday nor
did it state that she was a minor when she became involved in the
conspiracy. Additionally, the district court could not base its conclusions
regarding Ingham’s age and her involvement in the conspiracy on
information to which Flores was not privy. See Appellant’s Opening Br.
at 34 (noting that defense counsel did not have access to Ingham’s sealed
PSR). During oral argument, the Government claimed that Ingham’s
Sentencing Memorandum states that her birthday is July 9, 1991. This
may be true, but even if that memorandum were part of the record in this
case, whether Ingham’s birthday was in June or July 2009 is immaterial,
because there is no evidence in the record that establishes by a
preponderance that Ingham participated in the conspiracy before July
2009.
                 UNITED STATES V . FLORES                    23

opposed to merely linking her to Flores—was law
enforcement’s controlled purchase from Ingham in September
2009, which occurred after June 2009, the month Flores
testified that Ingham’s eighteenth birthday took place. In
addition, the PSR does not, contrary to the Government’s
assertion, state that Ingham was seventeen when she “became
involved in the conspiracy,” only that she was seventeen
when she became involved with Oxycontin and met Flores.
Ingham’s addiction to OxyContin and her relationship with
Flores when she was seventeen do not demonstrate that
Ingham joined the conspiracy at that time. Therefore, we
conclude that the Government did not meet its burden of
establishing that Ingham was a minor when she participated
in conspiracy.

     When a party raises a specific, non-frivolous argument
that is relevant to sentencing, “the judge should normally
explain why he accepts or rejects the party’s position.”
Carty, 520 F.3d at 992–93 (citation omitted). The district
court’s failure to do so is not procedural error where
“adequate explanation” may “be inferred from the PSR or the
record as a whole.” Id. at 992. The factual basis for the
district court’s imposition of the enhancement here cannot be
inferred from the PSR or the record as a whole. See Gall,
552 U.S. at 50 (“[A court] must adequately explain the
chosen sentence to allow for meaningful appellate review
 . . . .”). The district court provided no factual basis for its
conclusion and failed to resolve a factual dispute raised by
Flores. The facts in the PSR are inconclusive as to whether
Ingham was a minor when she joined the conspiracy, and the
trial record does not illuminate the issue. On this limited
evidentiary record, we cannot conclude that this enhancement
was supported by a preponderance of the evidence. Thus, we
must conclude that the district court abused its discretion by
24               UNITED STATES V . FLORES

imposing the enhancement without providing an adequate
factual basis.

                               C

    The Supreme Court has held that improperly calculating
the Guidelines range may constitute “significant procedural
error.” Gall, 552 U.S. at 51. Thus, even where, as here, the
district court imposes a sentence well below the Guidelines
range, it is difficult, if not impossible, to determine whether
the court would impose the same sentence if it were to heed
its imperative to keep the Guidelines range “in mind
throughout the process.” Carty, 520 F.3d at 991; Gall,
552 U.S. at 50 n.6 (“[D]istrict courts must begin their analysis
with the Guidelines and remain cognizant of them throughout
the sentencing process.”); United States v. Munoz-Camarena,
631 F.3d 1028, 1031 (9th Cir. 2011) (declining to find
harmless a district court’s incorrect application of an
enhancement, even though the district court sentenced the
defendant above the Guidelines range and stated it would
have imposed the sentence no matter the Guidelines
calculation, because the Ninth Circuit was not convinced that
the district court would impose the same sentence if the
correct Guidelines range were “kept in mind throughout the
process”) (citation omitted).

    In this case, the district court calculated Flores’s total
offense level as 42, which resulted in a Guidelines range of
360 months to life. Removing the incorrect two-level
enhancement would result in a correct total offense level of
40, with an associated Guidelines range of 292 to 365
months. Although the two Guidelines ranges overlap slightly
and the district court sentenced Flores to 180 months for the
drug charges, which is below both the original and the correct
                  UNITED STATES V . FLORES                     25

Guidelines ranges, we cannot say whether the district court
would impose the same sentence if it kept the correct
Guidelines range in mind throughout the process. See
Munoz-Camarena, 631 F.3d at 1031. Because we conclude
that material errors affected the district court’s Guidelines
calculation that served as the starting point for its sentencing
decision, we must vacate Flores’s sentence and remand this
matter for resentencing. Because we remand on a procedural
error, we decline to reach Flores’s argument regarding the
substantive reasonableness of his sentence. See Gall,
552 U.S. at 51 (holding that “the appellate court must . . . first
ensure that the district court committed no significant
procedural error” and should consider whether the sentence
is substantively reasonable only if “the district court’s
sentencing decision is procedurally sound”); Kilby, 443 F.3d
at 1140 (“If there is a material error in the Guidelines
calculation, we will remand for resentencing, without
reaching the question of whether the sentence as a whole is
reasonable . . . in light of the factors specified in 18 U.S.C.
§ 3553(a).”).

                               D

     Finally, the parties dispute whether we should allow the
district court to consider new evidence on remand. The
default rule is that “if a district court errs in sentencing, we
will remand for resentencing on an open record—that is,
without limitation on the evidence that the district court may
consider.” United States v. Matthews, 278 F.3d 880, 885–86
(9th Cir. 2002) (en banc) (“On remand, the district court
generally should be free to consider any matters relevant to
sentencing, even those that may not have been raised at the
first sentencing hearing, as if it were sentencing de novo.”
(citing United States v. Ponce, 51 F.3d 820, 826 (9th Cir.
26               UNITED STATES V . FLORES

1995); United States v. Caterino, 29 F.3d 1390, 1394 (9th
Cir. 1994))). We may depart from this general rule where
“‘additional evidence would not have changed the outcome
or where there was a failure of proof after a full inquiry into
the factual question at issue.’” Culps, 300 F.3d at 1082
(quoting Matthews, 278 F.3d at 886); see also United States
v. Reyes-Oseguera, 106 F.3d 1481, 1484 (9th Cir. 1997)
(remanding for resentencing on a closed record where
government offered insufficient evidence to support a
reckless endangerment enhancement).

    Here, we cannot conclude that there was a full inquiry
into the factual question at issue or that it would be futile to
introduce additional evidence regarding Ingham’s birthday
and her participation in the conspiracy. The Government did
not request an enhancement for use of a minor in its
sentencing memorandum. Rather, the enhancement was
recommended by the probation office and imposed sua sponte
by the district court at the sentencing hearing before the court
heard argument from the Government. The Government
stated it did not disagree with the district court that there was
evidence to support the enhancement, but did not present any
evidence in support of the district court’s finding. Under
these circumstances, we leave it to the district court’s
discretion to “assess the government’s right to submit further
evidence on this issue.” Jordan, 291 F.3d at 1029; see also
United States v. Gonzales, 207 F.3d 906, 914 (9th Cir. 2002)
(leaving the question whether to allow new evidence upon
resentencing to the district court’s discretion where the
court’s initial inquiry was incomplete). We remand to the
district court with directions to reconsider its sentencing
decision regarding the enhancement for use of a minor.

     VACATED and REMANDED for resentencing.
