Maria Elosu v. Middlefork Ranch Incorporated ( 2022 )


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  •                     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, District 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.