Maria Elosu v. Middlefork Ranch Incorporated

                    FOR PUBLICATION

   UNITED STATES COURT OF APPEALS
        FOR THE NINTH CIRCUIT


 MARIA FERNANDA ELOSU,                            No. 21-35309
 Individual; ROBERT LOUIS BRACE,
                Plaintiffs-Appellants,              D.C. No.
                                                 1:19-cv-00267-
                     v.                               DCN

 MIDDLEFORK RANCH
 INCORPORATED, an Idaho                             OPINION
 Corporation,
               Defendant-Appellee.


        Appeal from the United States District Court
                  for the District of Idaho
        David C. Nye, Chief District Judge, Presiding

          Argued and Submitted December 8, 2021
                   Pasadena, California

                    Filed February 23, 2022

Before: Carlos T. Bea and Kenneth K. Lee, Circuit Judges,
         and Richard D. Bennett, * District Judge.

                   Opinion by Judge Bennett

     *
       The Honorable Richard D. Bennett, United States District Judge
for the District of Maryland, sitting by designation.
2                ELOSU V. MIDDLEFORK RANCH

                          SUMMARY **


                        Expert Testimony

   The panel reversed the district court’s partial grant of
Middlefork Ranch Inc.’s motion to exclude expert testimony
and the subsequent entry of summary judgment in a diversity
negligence action, and remanded for further proceedings.

    Plaintiffs were the owners of a vacation cabin in Idaho
that burned. An expert report prepared by fire investigator
Michael Koster hypothesized that an open-flame pilot light
ignited combustible vapors from an oil stain on a wooden
deck and sparked the fire that burned the entire structure to
the ground. The district court excluded Koster’s testimony,
finding that the substance of his opinion was speculative,
uncertain and contradicted by multiple eyewitness accounts.

     The panel held that the district court improperly assumed
a factfinding role in this case. Although a court may screen
an expert opinion for reliability, and may reject testimony
that is wholly speculative, it may not weigh the expert’s
conclusions or assume a factfinding role. In the plain text of
its opinion, the district court took issue only with the expert’s
ultimate conclusions. In its findings, the district court
disregarded much of the expert’s scientific analysis, weighed
the evidence on record, and demanded corroboration –
factfinding steps that exceeded the court’s gatekeeping role.




    **
       This summary constitutes no part of the opinion of the court. It
has been prepared by court staff for the convenience of the reader.
               ELOSU V. MIDDLEFORK RANCH                    3

                        COUNSEL

Patrick C. Bageant (argued), Hollystone Law, Boise, Idaho,
for Plaintiffs-Appellants.

Gerald Kobluk (argued), KSB Litigation P.S., Spokane,
Washington, for Defendant-Appellee.


                         OPINION

BENNETT, Circuit Judge:

    On July 20, 2017, a fire swept through a cabin in the
Idaho wilderness. Nobody was at home, and neither
residents, neighbors, nor first responders saw the cabin catch
fire. Appellants Maria Elosu and Robert Brace contend that
the fire was caused by a negligent employee of Appellee
Middlefork Ranch, Inc. (“Middlefork”), the homeowners’
association which governs the cabin. Their case rises and
falls on an expert report prepared by fire investigator
Michael Koster, who hypothesized that an open-flame pilot
light on the northern end of the cabin ignited combustible
vapors from an excessive oil stain that had been applied to
the wooden deck the previous day—sparking a fast-moving
conflagration that swept across the deck and burned the
entire structure to the ground.

    This appeal arises from an order of the United States
District Court for the District of Idaho excluding Koster’s
testimony—and precluding Appellants’ case on the critical
element of causation, the sole triable issue that remained in
this litigation. The district court did not hold that Koster’s
methodology was unreliable or that he was not qualified to
render an expert opinion in the field of fire investigation.
4                ELOSU V. MIDDLEFORK RANCH

Indeed, the parties stipulated to his qualifications and
methodology. 1 Instead, the court took issue with Koster’s
“ultimate conclusions,” finding that the substance of his
opinion was speculative, uncertain, and contradicted by
multiple eyewitness accounts.

     District courts have a longstanding responsibility to
screen expert testimony, and to prevent unfounded or
unreliable opinions from contaminating a jury trial. See
generally Daubert v. Merrell Dow Pharm., Inc., 

509 U.S. 579, 586

 (1993). However, this Court has cautioned that the
district court is “a gatekeeper, not a fact finder.” Primiano v.
Cook, 

598 F.3d 558, 568

 (9th Cir. 2010) (quoting United
States v. Sandoval-Mendoza, 

472 F.3d 645, 654

 (9th Cir.
2006)). Although a district court may screen an expert
opinion for reliability, and may reject testimony that is
wholly speculative, it may not weigh the expert’s
conclusions or assume a factfinding role. We are compelled
to find that the district court assumed such a role in this case.
In the plain text of its opinion, the district court took issue
only with Koster’s “ultimate conclusions.” In its findings,
the court disregarded much of the expert’s scientific
analysis, weighed the evidence on record, and demanded
corroboration—factfinding steps that exceed the court’s
gatekeeping role. Accordingly, we reverse the judgment of



    1
        Those qualifications are extensive. Koster is a Fire/Arson
Investigator employed by Reliant Investigations, Inc., and has been
personally involved with 2400 fire investigations in 22 years of service
in the field. He has completed six courses and eleven training sessions
related to fire investigation and forensics, received multiple relevant
certifications, attended several conferences related to fire and arson
investigation, taught twenty-three courses, and been certified as an
expert witness in several prior cases.
                 ELOSU V. MIDDLEFORK RANCH                            5

the district court and remand for further proceedings
consistent with this opinion.

                        BACKGROUND

    Maria Elosu and Robert Brace, husband and wife, were
the owners and seasonal residents of Cabin 16, a luxury
vacation cabin located in the remote Valley County, Idaho,
and governed by Middlefork Ranch, Inc. Cabin 16 was a flat,
one-story building with a 500-square-foot wooden deck that
wrapped around the north, east, and south sides of the
structure. The northern end of the deck featured a propane-
fueled refrigerator atop an open-flame pilot light, with a
propane supply provided and managed by Middlefork. The
east side of the deck held various items of wooden
furniture—an old sofa, a teak picnic table, and benches. 2

    On the day before the fire, Brace power-washed and
stained the deck with Penofin-brand oil. Penofin oil is highly
flammable: Penofin oil cans come with detailed disposal
instructions and a warning label that specifically alerts the
user to the potential for “spontaneous combustion.”
Appellants’ chemical expert Douglas Byron concluded in his
report that Penofin oil is approximately as incendiary as
charcoal starter fluid—with a higher density than air and a
tendency to self-heat. When properly applied, Penofin oil
becomes dry to the touch after four hours, “serviceable” after
twelve, and fully cured after four to seven days. Based on the
application instructions, which dictate that one gallon should
be used for every 300 square feet of application, a 500-

    2
      Elosu also smokes cigarettes. The night before the fire, Elosu and
a guest smoked a cigarette on the deck, and ashes were found on the deck
the morning of the fire. Elosu smoked another cigarette “on the ground”
outside the following morning.
6                ELOSU V. MIDDLEFORK RANCH

square-foot deck should be treated with approximately one-
and-a-half gallons of Penofin oil. Both parties acknowledge
that Brace applied as much as two-and-a-half gallons that
day.

    On July 20, 2017, Appellant Brace left Cabin 16 prior to
8:00 a.m. He claims that he noticed sticky spots on the
wooden deck where the Penofin oil had failed to dry. Brace
provided rags for Elosu and their child to clean the deck,
instructing them to dispose of them in the fire pit. 3 Elosu
allegedly found the deck too sticky to clean. Thereafter, the
parties concur that a Middlefork employee visited Cabin 16
to check the propane levels, determined that the propane was
out or low, and elected to refill the propane and reignite the
pilot light. The parties also acknowledge that this employee
confirmed that the deck below the refrigerator was dry
before he lit the pilot light. Appellants further claim that the
deck remained sticky with oil, that “[Middlefork’s]
employee was standing in wet oil next to the refrigerator”
when he lit the flame, and that Elosu expressed concern that
the oil could ignite.

    Around 4:00 p.m., while Elosu and her child were away,
Cabin 16 caught fire. The first people to notice the fire were
a group of contractors working on a neighboring cabin,
including Kenny Pyle, Regee Rauch, and Greg Gamez.
These contractors did not see the fire ignite and arrived well
after the conflagration had spread. When they arrived, the
eastern deck was filled with flames, localized specifically
near the southeast corner. Rauch ran onto the north deck to

    3
       The parties dispute whether these rags were used and how they
were disposed of. One rag was entered into evidence, while two remain
unaccounted for. Appellants allege that they disposed of the rags in the
firepit.
                 ELOSU V. MIDDLEFORK RANCH                            7

bang on the windows and draw the attention of anyone
inside. Pyle attempted to extinguish the fire with a garden
hose, but retreated when the fire spread and engulfed the
entire structure. The cabin burned to the ground and was “a
complete and total loss.”

    Each party’s insurance company conducted initial
examinations of the scene: Appellants’ insurer sent Shane
Hargrove four days after the fire, while Middlefork’s insurer
sent Glen Johnson two months later. Hargrove and Johnson
each investigated the scene individually, before conducting
joint interviews of the eyewitnesses, each of whom
confirmed that there had not been a fire on the north-facing
deck when they arrived. 4 Johnson concluded that the fire had
originated on the southeast deck, and that it could have been
caused by discarded oil-soaked rags or a mop head. But
Johnson had no direct evidence of his theory, Hargrove
characterized the fire site as a “black hole,” and neither could
offer an opinion about the specific “source of ignition” that
rose to the level of “more probable than not.”

    Elosu and Brace retained three additional experts to
examine the fire site: Michael Koster, a fire investigator,
Richard Mumper, a mechanical engineer, and Douglas
Byron, a chemist. Mumper evaluated the remains of the
cabin and confirmed that there was no evidence of a
mechanical breakdown or another point of ignition. Byron
conducted a chemical analysis of Penofin oil in light of the
weather conditions on the day of the fire, and confirmed that
the Xylene vapors released by evaporating Penofin could

    4
       Middlefork exhaustively contends that Elosu and Brace pressured
these witnesses to change their testimony and confirm Appellants’ theory
that the fire originated on the north deck. These contentions were not
addressed by the district court and are not before us on appeal.
8                  ELOSU V. MIDDLEFORK RANCH

cause an explosion upon contact with an open flame. He also
determined that there was “no evidence or indication of
spontaneous combustion,” as witnesses had not smelled an
acrid odor, and video taken of the fire did not display the off-
white smoke characteristic of this phenomenon.

    Michael Koster, a fire investigator employed by Reliant
Investigations, Inc., was the only expert to offer a specific
theory as to the cause and origin of the fire. 5 Koster
personally examined the remains of Cabin 16 on May 27,
2018, evaluating the cabin’s wiring, structure, insulation,
and propane system to rule out external issues. 6 He retrieved
several five-gallon buckets of Penofin oil from a shed on the
property—one of which was half empty. He compared the
scene on the ground to photos of the cabin taken before the
fire to calculate “mass loss,” flame length, and rates of
exposure—concluding that the north end of the cabin had
seen “the most significant amount of mass loss.” He also
spoke with Brace to confirm his story of events, and
reviewed tapes of witness interviews from the previous
investigations.

    Koster reported his findings on January 10, 2020. In this
report, Koster reviewed literature on volatile liquids and
lumber fires, and applied professional standards governing
fire and explosion investigations, see National Fire
Protection Association, NFPA 921: Guide for Fire and

    5
      Mumper concurred with Koster’s theory as to the cause and origin
of the fire. However, the court granted Middlefork’s motion to exclude
this portion of Mumper’s report, reasoning that “Mumper is a mechanical
engineer, and appears to have been retained to opine on the mechanical
issues in this case, not the origins of the fire.” Elosu and Brace do not
challenge this ruling on appeal.

    6
        Koster’s examination was delayed by the winter frost.
               ELOSU V. MIDDLEFORK RANCH                      9

Explosion Investigations (2017 ed.) (“NFPA 921”), to
conclude that Cabin 16 had likely been destroyed by a fast-
moving vapor fire that was ignited by the refrigerator pilot
light. His hypothesis was that the excess Penofin oil Brace
applied to the deck had pooled beneath the floorboards,
evaporated during the heat of the following day, and ignited
upon encountering the open flame. The resulting
conflagration had swept through the deck, ignited the oil
stains and the furniture, and eventually consumed the entire
cabin.

    Koster reviewed Johnson’s report, and opined that it had
failed to comply with the NFPA 921 standards. Koster
challenged Johnson’s conclusion that the fire was caused by
discarded rags or a mop head, as “no remnants of cotton fiber
materials or rags were found . . . within his suspected area of
origination.” In a subsequent deposition, Koster also noted
that his opinion was consistent with the testimony of the
eyewitnesses, as the fire may have spread to the southeast
deck. Nevertheless, Koster also acknowledged that none of
the witnesses had substantiated his theory that the fire had
originated on the north side of the deck.

    Appellants filed suit against Middlefork on July 11,
2019. Consistent with Koster’s testimony, Appellants
alleged that Middlefork’s employee was negligent in
lighting the pilot light despite the fire hazard posed by the
cabin’s oil-stained deck. The case was reassigned to the
United States District Court for the Western District of
Washington to alleviate the District of Idaho’s heavy
caseload. One year later, a District Judge in Washington
denied the parties’ cross-motions for summary judgment and
transferred the case back to the District of Idaho for trial. On
October 23, 2020, Middlefork filed a Motion to Exclude
Expert Testimony, challenging all three of Appellants’
10             ELOSU V. MIDDLEFORK RANCH

experts. The district court granted this motion in part,
excluding Koster’s expert report and limiting the
admissibility of Mumper’s report. Thereafter, the parties
jointly stipulated to summary judgment, concurring that the
exclusion of Koster’s report had “eliminated a genuine
dispute of material fact as to the causation element of
[Appellants’] negligence claim.”

     This appeal followed.

                STANDARD OF REVIEW

    “We review evidentiary rulings for abuse of discretion
and reverse if the exercise of discretion is both erroneous and
prejudicial.” City of Pomona v. SQM N. Am. Corp., 

750 F.3d 1036, 1043

 (9th Cir. 2014) (citing Nev. Dep’t of Corr. v.
Greene, 

648 F.3d 1014, 1018

 (9th Cir. 2011)). This standard
applies on appeal from a motion to exclude expert testimony.
Kennedy v. Collagen Corp., 

161 F.3d 1226, 1227

 (9th Cir.
1998); Kumho Tire Co., Ltd. v. Carmichael, 

526 U.S. 137, 152

 (1999). The district court’s underlying factual
determinations are reviewed for clear error. United States v.
Lukashov, 

694 F.3d 1107, 1114

 (9th Cir. 2012).

                        ANALYSIS

    This case concerns the scope of a district court’s
discretion to exclude expert testimony that it deems
unsupported by the record. Rule 702 of the Federal Rules of
Evidence tasks a district court judge with “ensuring that an
expert’s testimony both rests on a reliable foundation and is
relevant to the task at hand.” Daubert, 

509 U.S. at 597

; see
also Daubert v. Merrell Dow Pharms., Inc. (Daubert II),

43 F.3d 1311, 1313

 (9th Cir. 1995). Rule 702 provides that
expert testimony is admissible if:
               ELOSU V. MIDDLEFORK RANCH                      11

        (1) the witness is sufficiently qualified as an
        expert by knowledge, skill, experience,
        training, or education; (2) the scientific,
        technical, or other specialized knowledge
        will help the trier of fact to understand the
        evidence or to determine a fact in issue;
        (3) the testimony is based on sufficient facts
        or data; (4) the testimony is the product of
        reliable principles and methods; and (5) the
        expert has reliably applied the relevant
        principles and methods to the facts of the
        case.

City of Pomona, 

750 F.3d at 1043

 (citing Fed. R. Evid. 702).

     This appeal brings the third element of Rule 702 into
tension with the rest. The parties concur that Koster is
qualified as an expert in fire investigation, that his testimony
is the product of reliable methodology, and that his report is
relevant to the critical fact at issue—the cause and origin of
the fire. Neither party contested these elements below, and
neither party argues them on appeal. Instead, this appeal
exclusively turns on whether Koster’s testimony was “based
on sufficient facts or data” as required by Rule 702. The
district court concluded that Koster’s report is too
speculative, that his conclusion conflicted with the
contractors’ testimony, and that he relied too heavily on the
testimony of the plaintiffs. This was an abuse of discretion,
as the district court assumed a factfinding role in its analysis.
These concerns speak to corroboration, not foundation, and
are properly addressed through impeachment before a jury
at trial—not exclusion by a district judge at the admissibility
stage.
12             ELOSU V. MIDDLEFORK RANCH

I. The District Court’s Gatekeeping Role

    As construed in Daubert v. Merrell Dow
Pharmaceuticals, Inc., Rule 702 tasks a district judge with
“ensuring that an expert’s testimony both rests on a reliable
foundation and is relevant to the task at hand.” 

509 U.S. at 597

. “Expert opinion testimony is relevant if the
knowledge underlying it has a valid connection to the
pertinent inquiry. And it is reliable if the knowledge
underlying it has a reliable basis in the knowledge and
experience of the relevant discipline.” Alaska Rent-A-Car,
Inc. v. Avis Budget Grp., Inc., 

738 F.3d 960, 969

 (9th Cir.
2013) (quoting Primiano, 

598 F.3d at 565

). To evaluate
reliability, the district court “must assess the expert’s
reasoning or methodology, using as appropriate criteria such
as testability, publication in peer-reviewed literature, known
or potential error rate, and general acceptance.” City of
Pomona, 

750 F.3d at 1044

. These factors are nonexclusive,
and “the trial court has discretion to decide how to test an
expert’s reliability . . . based on the particular circumstances
of the particular case.” 

Id.

 (quoting Primiano, 

598 F.3d at 564

).

    Ultimately, “the test under Daubert is not the correctness
of the expert's conclusions but the soundness of his
methodology.” Daubert II, 

43 F.3d at 1318

. The court is “a
gatekeeper, not a fact finder.” Primiano v. Cook, 

598 F.3d 558, 568

 (9th Cir. 2010) (quoting Sandoval-Mendoza,

472 F.3d at 654

). Accordingly, “[t]he district court is not
tasked with deciding whether the expert is right or wrong,
just whether his testimony has substance such that it would
be helpful to a jury.” Alaska Rent-A-Car, Inc., 738 F.3d
at 969–70. If the proposed testimony meets the thresholds of
relevance and reliability, its proponent is “entitled to have
the jury decide upon [its] credibility, rather than the judge.”
               ELOSU V. MIDDLEFORK RANCH                       13

Sandoval-Mendoza, 

472 F.3d at 656

. “Challenges that go to
the weight of the evidence are within the province of a fact
finder, not a trial court judge. A district court should not
make credibility determinations that are reserved for the
jury.” City of Pomona, 

750 F.3d at 1044

; accord Alaska
Rent-A-Car, Inc., 

738 F.3d at 969

 (“[T]he judge is supposed
to screen the jury from unreliable nonsense opinions, but not
exclude opinions merely because they are impeachable.”).
This Court has previously noted that “[s]haky but admissible
evidence is to be attacked by cross examination, contrary
evidence, and attention to the burden of proof, not
exclusion.” Primiano, 

598 F.3d at 564

.

    An expert’s specialized knowledge and experience can
serve as the requisite “facts or data” on which they render an
opinion. In Primiano v. Cook, a patient who had received an
elbow replacement sued the manufacturer of her prosthesis,
alleging that defects in the device’s polyethylene
components had caused joint problems that required several
additional surgeries to correct. 

598 F.3d at 562

. The patient
adduced testimony by Arnold-Peter Weiss, M.D., who
opined that polyethylene prostheses normally last between
five and fifteen years, and that it was unlikely plaintiff’s joint
issues were caused by “overuse, medical malpractice . . . or
other factors external to the device.” 

Id.

 at 562–63. The
district court granted the defendants’ motion to exclude this
testimony under Daubert, observing that Dr. Weiss had
failed to base his assumptions on an “objective source,” as
he had neither examined the plaintiff personally nor cited
any peer-reviewed publications that corroborated his
opinion. 

Id. at 563, 567

.

    This Court reversed, holding that the district court’s
concerns spoke to weight, not reliability. 

Id. at 568

. We
placed particular emphasis on Dr. Weiss’s experience and
14             ELOSU V. MIDDLEFORK RANCH

qualifications, the nature of medical testimony, and the
unusual issue at hand. 

Id. at 566

. As “medical knowledge is
often uncertain” due to the complexity of the human body
and the novelty of emerging medical issues, we reasoned that
“physicians must use their knowledge and experience as a
basis for weighing known factors along with the inevitable
uncertainties” to make “a sound judgment” in each case. 

Id.

at 565–66 (quoting Sandoval-Mendoza, 

472 F.3d at 655

).
Dr. Weiss’s “extensive, relevant experience” with the
implantation or revision of prosthetic elbows qualified him
to make such a judgment and provided a sufficient
foundation for his testimony. 

Id.

 “Given that the judge is ‘a
gatekeeper, not a fact finder,’ the gate could not be closed to
this relevant opinion offered with sufficient foundation by
one qualified to give it.” Id. at 568 (citation omitted).

    Relatedly, the requirement of “sufficient facts or data”
does not preclude an expert from making projections based
on reliable methodology. In Alaska Rent-A-Car, Inc. v. Avis
Budget Grp., Inc., featuring a breach of an exclusivity
agreement by a nationwide rent-a-car chain, the plaintiff
rental car company offered expert testimony to prove
damages. 

738 F.3d at 967

. This testimony required the expert
to “address a hypothetical world that never existed,”
extrapolating market projections based on similarly situated
rental companies to estimate the plaintiff’s lost profits. 

Id. at 968

. The defendant moved to exclude the expert’s opinion
as speculative, highlighting various market differences that
undercut his conclusions. 

Id.

 at 968–69. Rejecting this
assertion, the district court admitted this testimony, and we
affirmed, observing that Avis had not challenged the
expert’s credentials, qualifications, or methodology—only
the accuracy and credibility of his final projections. 

Id. at 970

. Each of the defendant’s countervailing considerations
were appropriate matters for impeachment, not
              ELOSU V. MIDDLEFORK RANCH                   15

admissibility. 

Id.

 at 969–70 (“[T]he judge is supposed to
screen the jury from unreliable nonsense opinions, but not
exclude opinions merely because they are impeachable.”).

    Finally, while a court may reject wholly speculative or
unfounded testimony, it abuses its discretion if it overlooks
relevant data submitted as the foundation of an expert’s
remarks. In Kennedy v. Collagen Corp., the plaintiff sued the
manufacturer of Zyderm, a facial smoothing product that
required subcutaneous injections, alleging that its
application had caused her to develop systemic lupus
erythematosus (“SLE”), “a debilitating and incurable
autoimmune disease.” 

161 F.3d at 1227

. The plaintiff
offered testimony by Dr. Joseph Spindler, who opined that
there was a link between Zyderm and SLE, relying on “peer-
reviewed publications and clinical studies” that suggested
Zyderm produced autoimmune antibodies. 

Id. at 1228

. The
district court excluded this testimony, noting that no
epidemiological studies had confirmed a link between
Zyderm and SLE, and that there was too great an “analytical
gap” between the data presented in the case and the expert’s
conclusion. 

Id.

 at 1229–30. And we reversed, observing that
“[t]he court did not consider all of the data relied upon by
Dr. Spindler, namely, studies by the defendant and others
finding that Zyderm can induce autoimmune reactions.” 

Id. at 1230

. Accordingly, “the gap was of the district court’s
making.” 

Id.

    The common thread running through these cases is that
Rule 702’s “sufficient facts or data” element requires
foundation, not corroboration. Consistent with the court’s
gatekeeping function, Rule 702 instructs a district court
judge to determine whether an expert “had sufficient factual
grounds on which to draw conclusions.” Damon v. Sun Co.,
Inc., 

87 F.3d 1467, 1475

 (1st Cir. 1996); Kennedy, 

161 F.3d 16

             ELOSU V. MIDDLEFORK RANCH

at 1228 (“[T]he focus of the inquiry envisioned by Rule 702
must be on the principles and methodology underlying an
expert’s testimony, not on the conclusions.”); see also City
of Pomona, 

750 F.3d at 1049

 (“A factual dispute is best
settled by a battle of the experts before the fact finder, not by
judicial fiat.”). Although “[a] court may conclude that there
is simply too great an analytical gap between the data and
the opinion proffered,” Gen. Elec. Co. v. Joiner, 

522 U.S. 136, 146

 (1997), Rule 702 does not license a court to engage
in freeform factfinding, to select between competing
versions of the evidence, or to determine the veracity of the
expert’s conclusions at the admissibility stage.

     This is consistent with the basic function of expert
testimony: to help the trier of fact understand highly
specialized issues that are not within common experience.
See Kumho Tire, 526 U.S. at 148–49, 156–57. Experts
working in specialized, scientific, and uncertain fields
regularly “extrapolate from existing data” and generate
novel hypotheses about complex issues. Joiner, 

522 U.S. at 146

. For this reason, “an expert is permitted wide latitude
to offer opinions, including those that are not based on
firsthand knowledge or observation.” Daubert, 509 U.S.
at 592–93. The court’s role is to determine “the scientific
validity” of an expert’s “principles and methodology,” not to
determine whether their hypothesis is correct, or to evaluate
whether it is corroborated by other evidence on the record.

Id.

 at 594–95. That is for the litigants to argue, and for the
jury to decide.

II. Evaluating Koster’s Proposed Expert Testimony

    Applying the principles articulated above, we must
determine whether the district court’s analysis of Koster’s
report exceeded the limited gatekeeping function
contemplated by Rule 702. In a 145-page fire origin and
               ELOSU V. MIDDLEFORK RANCH                   17

cause report applying the NFPA 921, Koster hypothesized
that the refrigerator pilot light ignited combustible vapors
produced by evaporated Penofin oil, producing a fast-
moving conflagration that swept along the deck and ignited
the stain on the southeast deck. Koster arrived at this
conclusion by conducting an analysis of the fire scene and
applying known scientific principles and established fire
investigation methodology. He studied the remains of the
cabin, examined evidence of fire movement and oxidation
patterns, and reviewed recorded interviews, deposition
transcripts, and video footage of the fire. He examined the
propane refrigerator and interviewed homeowners who were
present near the cabin in the days and hours leading up to the
fire. And he used before-and-after photographs to calculate
flame lengths and mass loss, referencing scientific literature
on the burning of milled lumber and volatile liquids.

    The district judge did not question Koster’s
qualifications or the reliability of his methodology. Instead,
the district court solely took issue with “his ultimate
conclusions,” adopting Middlefork’s contention that they
were “speculative and unsupported by the evidence.” In its
analysis of Koster’s proposed testimony, the court found that
only limited portions of his report were substantive, that the
underlying facts were susceptible to competing
interpretations, that he relied too heavily on the Appellants’
story of events, and that his conclusions conflicted with the
testimony of the eyewitnesses. This analysis exceeds the
district court’s limited gatekeeping role in several important
respects.

   First, the district court overlooked much of the scientific
analysis that formed the basis of Koster’s testimony. The
court concluded that only 20 to 25 pages of Koster’s 145-
page report were substantive in nature, distinguishing the
18             ELOSU V. MIDDLEFORK RANCH

remainder as “photographs, charts, and other supporting
documents.” But those “supporting documents” formed an
importing part of the “facts and data” that the district court
found lacking—including photographs and artifacts
obtained from the fire site, weather data, a forensics report,
and a reconstruction of the cabin. Accordingly, the same
error that demanded reversal in Kennedy is present here:
“Although the district court properly may exclude expert
testimony if the court concludes too great an analytical gap
exists between the existing data and the expert’s conclusion,
[this] gap was of the district court’s making.” 

161 F.3d at 1230

. A court cannot exclude expert testimony for lacking
“sufficient facts or data” while openly disregarding the
foundation of the expert’s opinion.

    Second, throughout its opinion, the district court
weighed the evidence and discredited Koster’s “ultimate
conclusions.” The court emphasized that Koster’s theory
was “directly contradicted by multiple eye-witness
accounts” of the fire, as the contractors had seen a fire raging
on the southeast deck, and Rauch had stepped onto the
northern deck to attempt to alert the occupants of the cabin.
The court took issue with Koster’s reliance on Brace’s
“interested” testimony, noting that Brace had hired Koster to
perform his analysis, and that Koster had made several
important assumptions “without conducting any
independent investigation.” And it weighed Koster’s report
against Johnson’s, observing that Middlefork’s expert had
“based his conclusions on other independently verifiable
facts and supporting evidence.” As Appellants note, this
analysis “fixat[ed] on evidence not offered in support of
[Koster’s] opinion while simultaneously ignoring the
evidence advanced on its behalf,” and exceeded the scope of
the Rule 702 inquiry. Cf. Daubert II, 

43 F.3d at 1318

 (“[T]he
               ELOSU V. MIDDLEFORK RANCH                    19

test under Daubert is not the correctness of the expert's
conclusions but the soundness of his methodology.”).

    As an initial matter, these factual conclusions appear to
be clearly erroneous. See Lukashov, 

694 F.3d at 1114

.
Contrary to the district court’s contention that Koster did not
conduct an “independent investigation,” Koster’s report
details his exhaustive personal examination of the fire scene,
studying factors such as mass loss, wind flow, fire spread,
and material heat release rates. His theory was also fully
consistent with the eyewitness testimony. Although there
was no fire on the northern deck when the contractors
arrived, the court’s singular focus on this fact misses the
point of Koster’s hypothesis: his theory was that an open-
flame pilot light caused flammable vapors to ignite a
sustained fire on the southeast deck before the first
responders witnessed the fire. Cf. NFPA 921 § 19.3.1.5
(“Gases, vapors, and combustible dusts . . . can cause
confusion about the location of the point of origin, because
the point of ignition can be some distance away from where
sustained fire starts in the structure or furnishings.”).
Accordingly, it was clear error for the district court to
construe Koster’s testimony as incompatible with the
statements of witnesses on the scene. Whether the first
responders initially saw fire on the north deck is not
dispositive, as they arrived after the fire had begun and did
not witness the moment of ignition.

    Given this logic, it is also notable that the court found
Johnson’s report admissible. Johnson theorized that the oil
on the deck had spontaneously combusted due to discarded
oil-soaked rags or an oil-soaked mop head. The district court
distinguished these theories from Koster’s by noting “that
Johnson based his conclusions on other independently
verifiable facts and supporting evidence.” But Middlefork
20             ELOSU V. MIDDLEFORK RANCH

acknowledges that none of the experts investigating the
scene could account for the oil-soaked rags, and Johnson
conceded there was no “direct evidence” of his theory.
Accordingly, Johnson’s testimony is no more “concrete”
than Koster’s. It was clear error to conclude otherwise, and
it was an abuse of discretion to select between these experts’
competing versions of events. Cf. City of Pomona, 

750 F.3d at 1049

 (“Where two credible experts disagree, it is the job
of the fact finder, not the trial court, to determine which
source is more credible and reliable.”).

    In any event, these concerns are matters for
impeachment, not admissibility. In performing its
gatekeeping function, a district court “is not tasked with
deciding whether [Koster] is right or wrong, just whether his
testimony has substance such that it would be helpful to a
jury.” Alaska Rent-A-Car, 738 F.3d at 969–70. NFPA 921
instructs fire investigators to rely on the observations of
witnesses and property owners when determining the origin
and cause of a fire. See NFPA 921 §§ 18.3.3.14, 19.3.1.6,
19.3.3. Koster acted in accordance with this professional
guideline when he relied on Brace’s opinion, which was
relevant to the central issue of the case: Brace applied the
Penofin oil stain to the deck and was one of the last people
to observe the deck before the cabin caught fire. It may well
be that Koster relied too heavily on an “interested party,”
that his report was not sufficiently corroborated, or that he
was biased towards Appellants, financially or otherwise. As
in Alaska Rent-A-Car, these countervailing concerns “go to
the weight of the testimony and its credibility, not its
admissibility.” 

738 F.3d at 970

; accord Primiano, 

598 F.3d at 564

 (“Shaky but admissible evidence is to be attacked by
cross examination, contrary evidence, and attention to the
burden of proof, not exclusion.”).
               ELOSU V. MIDDLEFORK RANCH                   21

     Finally, the court erred by demanding “concrete physical
or testimonial evidence” in a field characterized by
uncertainty. The court acknowledged that “Koster was able
to point to various factors that influenced his opinion,” but
found that “each of those factors have numerous alternative
explanations that could lead to alternative outcomes as
well.” By way of example, the court noted that the oxidation
Koster observed on the north side of the cabin was consistent
with the presence of strong north-to-south winds on the day
of the fire, that none of the witnesses he had interviewed
suggested that the point of ignition was on the north deck,
and that Koster lacked data supporting his theory that
Penofin oil had pooled beneath the deck on the following the
stain. The court concluded that the presence of “competing
interpretation[s]” and the lack of “concrete physical or
testimonial evidence” in support of Koster’s theory rendered
it too speculative to be admissible.

    Quite simply, Koster is a fire investigator. The fact that
his testimony relied on circumstantial evidence and
inferences is neither unusual nor unexpected, as fires
routinely destroy all evidence of their origins. “By the very
nature of a fire, its cause must often be proven through a
combination of common sense, circumstantial evidence and
expert testimony.” Ricci v. Alt. Energy Inc., 

211 F.3d 157

,
162–63 (1st Cir. 2000) (quoting Minerals & Chems. Philipp
Corp. v. S.S. Nat’l Trader, 

445 F.2d 831

, 832 (2d Cir.
1971)); Hartley v. St. Paul Fire & Marine Ins. Co., 

118 F. App’x 914, 919

 (6th Cir. 2004) (quoting same).
Accordingly, fire investigation, no less than medicine,
requires sound judgment in the face of uncertainty. Cf.
Primiano, 

598 F.3d at 566

. An expert in either field must
“use their knowledge and experience as a basis for weighing
known factors along with the inevitable uncertainties.” Id.;
Joiner, 

522 U.S. at 146

 (noting that experts regularly
22             ELOSU V. MIDDLEFORK RANCH

“extrapolate from existing data”); Daubert, 

509 U.S. at 592

(emphasizing that an expert need not testify based on
“firsthand knowledge or observation”). The court’s
responsibility is to ensure that a sufficiently qualified expert
applied reliable principles to form their hypothesis—not to
gauge whether that hypothesis is ultimately correct. That is
for the litigants to prove, and for the factfinder to decide.

    It is undisputed that Koster was qualified as an expert in
fire investigation, and that he applied broadly accepted
scientific principles and professional standards to conduct
his analysis. Koster is a Fire and Arson Investigator
employed by Reliant Investigations, Inc., with multiple
certifications and twenty-two years of experience in the
field. He reached his conclusions by applying NFPA 921,
which has been consistently accepted as a suitable
foundation for fire investigation testimony. See Schlesinger
v. United States, 

898 F. Supp. 2d 489, 504

 (E.D.N.Y. 2012)
(collecting cases); McCoy v. Whirlpool Corp., 

214 F.R.D. 646, 653

 (D. Kan. 2003) (calling NFPA 921 “gold standard”
for fire investigations). Those guidelines provide direct
support for Koster’s vapor fire theory. See, e.g., NFPA 921
§ 19.1.3 (noting that vapor fires often ignite far from the
location of the first sustained heavy fire and leave “no
physical evidence of an ignition source” at the origin of the
fire.). They also instruct fire investigators to rely on the
precise kinds of circumstantial evidence that formed the
basis of Koster’s opinion. See, e.g., NFPA 921 § 18.1.2
(directing investigators to examine witness statements, fire
patterns, arc mapping, and fire dynamics). Accordingly, “the
gate could not be closed to this relevant opinion offered with
sufficient foundation by one qualified to give it.” Primiano,

598 F.3d at 568

.
              ELOSU V. MIDDLEFORK RANCH                   23

                     CONCLUSION

    We REVERSE the district court’s partial grant of
Middlefork’s motion to exclude expert testimony and the
subsequent consent entry of summary judgment, and
REMAND this case to the United States District Court for
the District of Idaho for further proceedings consistent with
this opinion.