Challenging an Expert's Methodology

Credentials are the weakest ground and the most commonly argued

A challenge that opens by comparing curricula vitae is usually a challenge that fails. A witness whose credentials are less impressive than the opposing expert’s is still qualified, and the Tenth Circuit’s formulation — drawn from Compton v. Subaru of America, Inc., 82 F.3d 1513 (10th Cir. 1996) — is that the question is whether the issue falls “within the reasonable confines” of the expert’s subject area. A mere lack of specialization goes to weight.

The qualification argument wins where the opinion is outside those confines. In Ralston v. Smith & Nephew Richards, Inc., 275 F.3d 965 (10th Cir. 2001), the court affirmed exclusion of an orthopedic surgeon’s opinion on the adequacy of warnings accompanying an intramedullary nail: her general orthopedic qualifications did not reach the specific subject, on which she had conducted no research and with which she was not familiar. In Gayton v. McCoy, 593 F.3d 610 (7th Cir. 2010), a physician’s general medical qualifications supported some causation opinions and not others, and the court parsed them.

That points to the structural fact underneath the whole subject: qualification is assessed opinion by opinion, not witness by witness. Courts admit part of an expert’s testimony and exclude the rest far more often than they exclude a witness outright, and a challenge drafted opinion by opinion is asking for something a court is used to granting.

The defect most successful challenges are built around

General Electric Co. v. Joiner, 522 U.S. 136 (1997), supplies both the standard of review and the language that most methodology challenges are written toward: “[N]othing in either Daubert or the Federal Rules of Evidence requires a district court to admit opinion evidence that is connected to existing data only by the ipse dixit of the expert. A court may conclude that there is simply too great an analytical gap between the data and the opinion proffered.” 522 U.S. at 146.

The facts matter as much as the sentence. The plaintiff’s experts relied on studies of infant mice injected with massive doses of PCBs directly into the peritoneum, and on epidemiological studies whose own authors had declined to draw a causal inference, to support an opinion that workplace exposure had promoted the plaintiff’s small-cell lung cancer. The Court held the district court had not abused its discretion in finding the gap too great.

Joiner also disposed of a line that is still quoted out of context. Daubert had said in 1993 that the focus “must be solely on principles and methodology, not on the conclusions that they generate,” 509 U.S. at 595. Four years later Joiner qualified it: “conclusions and methodology are not entirely distinct from one another.” Quoting the 1993 sentence alone states the law as it stood for four years, three decades ago. Opinion text: Joiner, 522 U.S. 136.

The failures that actually get opinions struck

Published exclusions cluster into a small number of recognizable patterns.

• Selective use of data. In In re Onglyza (Saxagliptin) and Kombiglyze XR Products Liability Litigation, 93 F.4th 339 (6th Cir. 2024), the court affirmed exclusion of a general-causation expert whose opinion rested heavily on a single finding within one trial while discounting contrary evidence. In EEOC v. Freeman, 778 F.3d 463 (4th Cir. 2015), the concurrence described a report containing a “plethora” of “analytical fallacies” and observed that cherry-picking data produces a “misleadingly favorable result by looking only to ‘good’ outcomes.”
• A method assembled for the case. In Elcock v. Kmart Corp., 233 F.3d 734 (3d Cir. 2000), a vocational expert combined two accepted approaches into what the court called “nothing more than a hodgepodge” of them, and then produced a disability figure his own stated methodology would not have generated.
• Assumptions unsupported by, or contrary to, the record. The economist in the same case assumed annual earnings of $12,480 where the plaintiff had earned $5,774, and 100 percent disability against the vocational expert’s own 50 to 60 percent.
• Obvious alternative causes not ruled out. Tamraz v. Lincoln Electric Co., 620 F.3d 665 (6th Cir. 2010), reversed the admission of a specific-causation opinion whose differential etiology was not reliably performed.
• Overstatement beyond what the method supports. The 2023 Advisory Committee Note addresses this directly, warning forensic experts against assertions of absolute certainty where the methodology is subjective and subject to error.

Reliance is not a license to assume facts

FRE 703 permits an expert to base an opinion on facts or data that would themselves be inadmissible, provided experts in the particular field would reasonably rely on those kinds of facts or data. The distinction in that sentence carries more weight than it is usually given: the rule is about the type of material, not about whether an assumed fact is true.

An expert may therefore rely on a body of literature, on industry data, or on another witness’s work product without those materials being independently admitted — and may not rely on an assumption the record contradicts. Elcock is the clearest published illustration of the difference, because the economist’s inputs were not exotic; they were simply wrong on the record before the court.

A related failure is the expert who did not do the work. Where the analysis was performed by staff and the named witness cannot explain it, the opinion is exposed under 702(b) and 702(d), and separately under FRCP 26(a)(2)(B), which requires the report to be prepared and signed by the witness. This is a line of questioning that produces its material at deposition or not at all.

Weight or admissibility, and why the line is still moving

The most consequential development in this area is the relocation of the boundary between weight and admissibility. Effective 1 December 2023, Rule 702 requires the proponent to demonstrate each condition to the court by a preponderance, and the Advisory Committee explained why: “many courts have held that the critical questions of the sufficiency of an expert’s basis, and the application of the expert’s methodology, are questions of weight and not admissibility. These rulings are an incorrect application of Rules 702 and 104(a).”

The category was narrowed, not abolished. Ordinarily still weight: a less impressive resume; a well-qualified opposing expert who reached a different conclusion; a contrary study the expert acknowledges and addresses; a judgment call within an accepted method; compensation and retention history; a minor computational error not driving the result. Ordinarily admissibility after 2023: an opinion outside the field the witness actually has; a stated methodology not applied to these facts; facts or data insufficient to support the opinion at all; selective use of data that drives the result; a method that exists only for this litigation; a conclusion connected to the data only by the expert’s say-so.

Courts of appeals have been visibly re-sorting decisions into the second column — among them Sprafka v. Medical Device Business Services, 139 F.4th 656 (8th Cir. 2025), which rejected the circuit’s own long-standing rule that the factual basis of an expert opinion goes to credibility rather than admissibility, and EcoFactor, Inc. v. Google LLC, 137 F.4th 1333 (Fed. Cir. 2025) (en banc).

The Supreme Court was asked to settle it and declined

The re-sorting is not complete, and there is a published decision going the other way. In Sommerville v. Union Carbide Corp., 149 F.4th 408 (4th Cir. 2025), the Fourth Circuit reversed an exclusion under Rule 702(b) and held that “questions regarding the factual underpinnings of the [expert] opinion affect the weight and credibility of the witness’ assessment, not its admissibility,” over a dissent from the Chief Judge. Rehearing en banc was denied.

The defendants petitioned for certiorari as Union Carbide Corp. v. Sommerville, No. 25-919, presenting the question whether challenges to an expert’s factual basis always go to weight under Rule 702, or instead go to admissibility where the court first finds the evidentiary support insufficient. The petition was docketed on 4 February 2026 and denied on 26 May 2026, with Justice Alito taking no part.

A denial of certiorari is not a ruling on the merits and decides nothing. What it means practically is that as of mid-2026 the post-2023 division over whether a factual-basis challenge goes to weight or to admissibility remains unresolved by the Supreme Court, with the Fourth Circuit an outlier and several other circuits moving the other way. For a party briefing the question, the circuit’s own post-December-2023 authority is what matters, and pre-amendment circuit precedent holding that basis or application is always a weight question should be checked before it is relied on by either side. Docket: No. 25-919.

The experience-based expert takes a different attack

Kumho Tire Co. v. Carmichael, 526 U.S. 137 (1999), brought technical and other specialized knowledge inside the gatekeeping obligation, and it also foreclosed the laziest way of using that fact. The reliability factors identified in Daubert “neither necessarily nor exclusively appl[y] to all experts or in every case,” so demonstrating that an engineer, appraiser or industry-practice witness has no error rate and no peer-reviewed publication is not a ground for exclusion.

The operative test for that witness is whether the expert “employs in the courtroom the same level of intellectual rigor that characterizes the practice of an expert in the relevant field.” The productive challenge is correspondingly different: what does the witness do outside litigation when answering this kind of question, was that done here, and what closes the distance between the experience relied on and the conclusion offered?

The 2000 Advisory Committee Note supplies five additional considerations that fit this witness better than the Daubert list — whether the opinions grew out of independent research or were developed for testifying, whether the expert extrapolated unjustifiably, whether obvious alternative explanations were accounted for, whether the expert was as careful as in regular professional work, and whether the field is known to reach reliable results for the type of opinion offered. They are an Advisory Committee compilation of lower-court practice rather than Supreme Court holdings, and should be cited as such.

What a challenge is worth when it does not succeed

Exclusion is the exception, and a challenge is not wasted when it fails. The realistic objectives, in descending order of frequency, are narrowing the opinion, obtaining a limiting instruction, forcing commitments in the opposition brief that constrain the trial testimony, generating the cross-examination, and only then exclusion.

Each of those has a mechanism behind it. The briefing forces the proponent to state in writing what the methodology actually is, and that statement is available at trial when the testimony drifts from it. A hearing produces sworn testimony under focused questioning that a deposition may not have produced. And the judge who denies the motion has nonetheless read a detailed account of the weaknesses before hearing the witness. None of that is available to a party that saves everything for cross-examination.

Daubert supplies the sentence that opposition briefs quote most: “Vigorous cross-examination, presentation of contrary evidence, and careful instruction on the burden of proof are the traditional and appropriate means of attacking shaky but admissible evidence.” 509 U.S. at 596. It remains good law, and the operative words are shaky but admissible — the sentence describes the remedy for testimony that has already cleared the rule, not a presumption that it will.

Frequently Asked Questions

Published 2026-08-12