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A Tougher Look at Expert Testimony

Catherine Milling


  • The 2023 amendments to Federal Rule of Evidence 702 clarify and refine the standards for expert witness testimony.
  • The amendments make it clear that it is the court's responsibility to assess the admissibility of expert testimony based on its reliability, prior to it reaching a jury, by a preponderance of the evidence.
  • The changes specify that the party presenting the expert testimony must demonstrate that the testimony meets Rule 702’s requirements, shifting away from any presumption of admissibility.
  • Legal practitioners must now be more diligent in selecting and presenting expert witnesses, ensuring their methodologies and the application to the facts are robust and defensible.
A Tougher Look at Expert Testimony
Semen Salivanchuk via Getty Images
It’s not what you know; it’s what you can prove in court.

—Denzel Washington, Training Day (2001).

When it comes to proving the case to the finder of fact, expert witnesses play an immense role in equipping litigators with the necessary evidence and opinions to effectively present their side. Experts provide the sort of complex, specialized explanations lawyers cannot convey to the jury on their own. 

For those not already familiar with it, Federal Rule of Evidence 702 governs testimony by expert witnesses. In 2022, after five years of discussion and research, the Advisory Committee on Evidence Rules approved a set of proposed amendments to Rule 702. The changes went into effect on December 1, 2023, following the U.S. Supreme Court’s adoption of the proposed amendments. The last substantive amendment to the rule occurred over two decades ago, following the decision in Daubert v. Merrell Dow Pharmaceuticals, Inc., and its progeny. The 2023 amendments clarify sections of the rule consistently misapplied and misstated by courts for several years. The amended version of Rule 702 (new language underlined, deleted language omited) states:

Rule 702 Testimony by Expert Witnesses

A witness who is qualified as an expert by knowledge, skill, experience, training, or education may testify in the form of an opinion or otherwise if the proponent demonstrates to the court that it is more likely than not that:

(a)   the expert’s scientific, technical, or other specialized knowledge will help the trier of fact to understand the evidence or to determine a fact in issue;

(b)  the testimony is based on sufficient facts or data;

(c)   the testimony is the product of reliable principles and methods; and

(d) the expert’s opinion reflects a reliable application of the principles and methods to the facts of the case.

While the amendments are not intended to drastically overhaul expert witness practice, lawyers should be aware of the amendments and the additional commentary provided by the Advisory Committee’s report and note, and should educate themselves on the extent the change affects their practice.

Admissibility—The Court as Gatekeeper

Prior to the release of the most recent version of Rule 702, practitioners and courts alike questioned whose role it was to determine the reliability of an expert’s opinion. Did an expert’s qualifications to testify go to the weight of the evidence for the jury or to admissibility for the judge? The question divided courts, leading many to misapply the rule for years.

The amended language “the proponent demonstrates to the court that it is more likely than not . . .” makes clear that the court must decide that the testimony, more likely than not, meets the reliability requirements of Rule 702. The court’s gatekeeping function provides practitioners with a line of defense against inadequate scientific and expert opinions and allows the court to sift through the complex analysis of an expert’s reliability before the testimony reaches the jury.

The amended rule also clarifies what standard of proof applies to the admissibility of expert testimony, another aspect frequently misapplied over the years. The court must make the determination that the questioned testimony satisfies Rule 702’s reliability standards by a preponderance of the evidence (more likely than not).

Take note of what the amended rule is not saying. It is not the court’s role to determine which opinion is correct when experts come to differing conclusions. So long as the court finds that the expert’s testimony is admissible, further challenges to the opinion will attack the weight of the evidence. In addition, the court need not make an admissibility determination in the absence of a challenge, so practitioners should not rely on the judge to question the admissibility of testimony without first submitting proper motions.

Advocacy—The Proponent’s Burden

Confusion has surrounded whether or not an expert’s opinion is afforded an automatic assumption of admissibility that the challenger must overcome. The amended language “. . . the proponent demonstrates . . . ,” along with the advisory committee’s commentary, makes clear that expert testimony is not presumed to be admissible. The proponent of the testimony must affirmatively prove that the testimony meets all of the required elements of Rule 702 by a preponderance of the evidence. As is true for the reliability requirements discussed above, it is still incumbent upon an opposing party to properly challenge the testimony before the burden shifts to the proponent.

Reliability—The Facts Matter

The most notable difference is seen in Rule 702(d), modified from “the expert has reliably applied the principles and methods to the facts of the case” to “the expert’s opinion reflects a reliable application of the principles and methods to the facts of the case.” The challenge facing an expert’s testimony is clearly twofold. The opposition may question both an expert’s methodology and whether the testimony is an accurate reflection of that methodology. An expert witness must be capable of articulating the methods used when coming to his or her conclusions and how those methods actually applied to the facts of the case at hand.

Furthermore, the advisory committee’s commentary highlights the dangers of an expert overstating his or her opinion. Before introducing testimony, confirm that the expert’s applied methodology supports every portion of the opinion, and be wary of any opinions that tout 100 percent success rates or brand themselves as fail-proof.

Application—How Does This Affect You?

The question you should ask yourself whenever a rule change occurs is, of course, what will you need to do to comply? While not an exhaustive list, below are a few thoughts to consider in light of the recent amendments:

  • If you practice in a jurisdiction where courts previously misapplied the rule, it is imperative to provide the court with the proper briefing on the amendments and the accompanying commentary whenever a party challenges expert testimony. It is possible that you will be one of the first practitioners litigating the issue under the amended rule. Equip the court with the knowledge it needs.
  • Be mindful of the interplay between your state’s rules of evidence and the Federal Rules of Evidence. The amendments may significantly affect your state court practice, and it may be advisable to brief your state court judges on the changes, as well.
  • The best defense is a good offense. From the very first time you read an expert’s curriculum vitae, think critically about whether this person is actually going to be able to prove what is necessary. How is the expert going to prove it? Is the expert actually qualified to give this opinion? A strong understanding of an expert’s particular area of expertise on the front end may help ward off a challenge down the road.
  • Assess whether every piece of an expert’s testimony results from the sufficient application of reliable methodology. Avoid a situation where opposing counsel can successfully bring a partial challenge because an expert went out on a limb with some portion of his or her opinion.
  • Do not delay in ensuring expert witnesses are in compliance with the amended Rule 702, as courts are already relying on the amendments and the advisory committee’s commentary to guide their decisions and overturn erroneous rulings.
  • Use the amendments as both a sword and shield. When deposing and cross-examining opposing expert witnesses, home in on their methodology and whether it supports each portion of their opinion. When assessing your own expert’s opinions, think critically about the opinions presented and what basis supports the conclusions.

A strong understanding of amended Rule 702 will equip you with the knowledge to successfully work with experts and present the court with the correct and most up-to-date authority. Ensure that your team, clients, and retained experts are familiar with amended Federal Rule of Evidence 702.