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A Guide to Rule 702 for Expert Witnesses

May 9, 2024
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By Noah Bolmer

This guide aims to clarify the most common rubric by which expert witnesses are evaluated for subject matter competence and application, Federal Rule of Evidence 702. Qualified expert witnesses should be aware of the factors that judges consider in admitting their testimony and prepare for any challenges. Similarly, attorneys will consider each prong of Rule 702 during the vetting process and should only consider hiring experts with the appropriate subject matter expertise given the specific facts of the case.


My advice to an attorney is to get an expert with the best credentials [. . .] you do not want to get into a situation where you have a bogus expert. When push comes to shove, they rip holes into your credentials, your honesty, and your evidence. You do not want to be in that position. –Dr. Hervey Segall

It would be unethical to accept an engagement unless you are actually an expert, and most states have implemented some variation of Federal Rule of Evidence 702 when determining whether an expert is suitable for a specific case.


In the landmark 1923 Frye v. United States ruling, the court held that “expert testimony must be based on scientific methods that are sufficiently established and accepted in the particular field in which it belongs” setting the stage for requiring a rigorous, proven basis for the admission of expert opinions. In certain states, this continues to be the standard of review, or basis thereof; including California, Minnesota, New York, Pennsylvania, and others.

In 1975, Congress passed the act which installed the Federal Rules of Evidence, including Rule 702 governing expert witness testimony. It read, in its entirety:

If scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education, may testify thereto in the form of an opinion or otherwise.

Squaring Frye with Rule 702 remained unresolved in Federal cases until Daubert v. Merrell Dow Pharmaceuticals, Inc. (1993), where the court ruled that Rule 702 supersedes Frye. It has been amended several times (last in 2023) to clarify guidelines for admitting expert witness testimony.

The amended Rule 702 now requires:

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.

The 2023 amendment adds “more likely than not” (sometimes known as the preponderance of the evidence standard, or the 50% rule) to the preamble, setting the standard of review. Each prong is now evaluated under this standard by the judge when determining the admissibility of expert testimony.

Even among states that still rely on the older Frye standard, their own standard, or something in-between, in a practical sense, the considerations are largely the same (although judges have considerably more latitude in Frye jurisdictions). An ethical expert witness should satisfy all criteria relevant to the case’s venue. Check with your engaging attorney to make sure you fully understand expert witness requirements in the action’s jurisdiction. Never accept an engagement where you feel unqualified, even if pressured to do so.


The preamble to Rule 702 requires that an expert be well-qualified, before considering each prong of the standard, and this is a common basis for an expert challenge. There are several routes by which subject matter experts become expert witnesses, as the first consideration in Rule 702, is actual knowledge through “skill, experience, training, or education”. Whether a pure academic, a seasoned professional, or commonly, a hybrid; it is important that you can demonstrate your credentials to the satisfaction of the court.

Many expert witnesses consider academia and credentials to be crucial differentiators, as they are tangible, verifiable and demonstrate knowledge in a field. Expert B.J. Hawkins notes “I think that no matter the professional organization, whatever continuing education credits that you have in your field and the field of the expert witness are important.” Attorney Stanley Gibson notes that credentials serve as a bulwark against potential challenges, “You need to get your expert through the Daubert process or through a motion to exclude in state court. So, you want to make sure they are qualified and the more credentials they have for that process [the better].”

On the other hand, experience can substitute for, or augment academic and industry credentials. Dr. David Williams recalls, “After one of my workshops, someone asked me, ‘What are your credentials and how many years of experience should you have to be an expert witness?’ and I said, ‘I think a minimum of 20 years of working experience.’ Hopefully, you have learned your stuff sufficiently so you can be cognizant of it and understand all the nuances of it.”

702(a) Prong: Relevance

Once your expertise in a field has been properly established, the next hurdle is whether that expertise directly applies to the case. Specifically, it must help the trier of fact (a jury, or judge in a bench trial) understand a piece of evidence or determine a fact in issue.

As an example, in a products liability case, a mechanical expert might be brought in to help determine whether a device malfunction was due to insufficient maintenance. However, an expert brought in to describe the basic operation of a common device would not satisfy the relevance prong, as a reasonable person would not require aid in understanding.

One of the most common ways that expert witnesses aid lay jurors or judges is in parsing technical jargon. Expert Erik Bernstein notes, “As expert witnesses, a big part of our job is to educate the folks who are reading or hearing our testimony. Making it so they can digest it and understand it without going to a dictionary or searching online is huge.”

Federal Rule 702(a) impacts ethics by outlining standards for expert testimony. By ensuring experts are qualified, and their testimony is based on sound reasoning and reliable methods, the rule promotes accuracy and fairness.

702(b) Prong: Sufficiency

Expert witnesses must present fully supported opinions, without slipping into speculation. Expert Ravi Iyer admonishes, “Do not try to speculate or guess something you do not feel comfortable about. It is okay to say, ‘I do not know,’ which is acceptable. That was a big a-ha moment for me. They asked me, and I did not know the answer. And I said, ‘I do not know.’ It is okay to be able to say that.”

Of course, forming a well-supported opinion requires attorneys to share adequate information with their experts. Expert David Harkavy explains, “Certainly, the lawyers trust the expert to perform the work at their direction and with objectivity and integrity. The expert trusts the lawyers to provide the expert with sufficient relevant information for the expert to perform a reliable and supportable [opinion] and protect the expert when opposing counsel goes after the expert.” Professor David Rockstraw notes, “I [always] want a copy of the complaint to understand the legal basis for the lawsuit.” While attorneys should strive to provide all necessary data at the outset, it is incumbent upon expert witnesses to inform the engaging attorney if there is insufficient basis to present an opinion.

702(c) Prong: Methodology

Experts are expected to use industry-standard, proven “principles and methods” in forming an opinion. If an expert relies on a new device or method, they will be open to challenges as to efficacy and application.

If an expert acquires new information specifically for use in the case at hand, it may be less persuasive than prior expertise. Similarly, attorneys who use an expert who relies on new, novel, or less-supported methodologies will have a more difficult time convincing the court to admit their testimony. Attorney Damian D. Capozzola observes (1):

If the expert’s opinion arises from litigation-related research, is not generally accepted among his peers, and has not undergone peer review and publication, the expert must [citing Daubert] “explain precisely how they went about reaching their conclusions and point to some objective source—a learned treatise, the policy statement of a professional association, a published article in a reputable scientific journal or the like—to show that [they have] followed the scientific method, as it is practiced by (at least) a recognized minority of scientists in [their] field.” In doing so, the expert must explain and defend each step in [their] proposed methodology because [citing State v. Foret] “any step that renders the analysis unreliable under the Daubert factors renders the expert’s testimony inadmissible.” This is true whether the step completely changes a reliable methodology, or merely misapplies that methodology.

702 (d) Prong: Application

Once a particular methodology is shown to be sound, it must be properly applied to the facts of the case. The judge acts as gatekeeper, preventing speculation or personal beliefs from influencing the jury under the guise of expertise.

There are differences among the federal circuits in how this is interpreted (the Ninth favors admission over exclusion more than the Third circuit, for example).

Forensics expert Michael Primeau describes the manner in which even new technology is standardized and codified in a repeatable way, so that any expert should come to the same result:

People smarter than us were testing [new] tools, looking at how proprietary surveillance is created, along with how to make them more readily available to courts, and make sure that was done accurately so agencies like SWDGE—the scientific group working on digital evidence—pushed to promote and publish those documents [. . .] I was recently asked on the stand, by the attorney I was working for “You know what? What is your opinion? What do you think?” I looked at him, the judge, and the prosecutor and said, “That is the best part about what I do. It does not matter what I think. It matters what the evidence says. It matters what the test result is. It does not put it on my shoulders. I am just interpreting that information for a trial, a judge, and a jury.” So, when technology evolves and we are on the front lines, like when there is an update with an app or there is a change in the way the camera works with the firmware, we test that information and have a known exhibit called an exemplar to compare. [. . .] It matters what the method says so that everyone can arrive at the same result. If someone like me has similar qualifications on the other side, we are going to arrive at the same result.

Expert Challenges

Expert witnesses may be challenged for a lack of expertise, or any other prong of Rule 702. While these are colloquially known as Daubert Challenges, there are three cases, often known as the Daubert Trilogy whose jurisprudence is often used to impeach or exclude expert witnesses. Similarly, in non-702 (or hybrid) venues, there may be other persuasive caselaw; but the holdings are broadly similar.

The Daubert Trilogy

In Daubert v. Merrell Dow Pharmaceuticals, Inc. (1993), the court held that Rule 703 is not an application of the Frye standard, rather, it is a standalone piece of legislation, and should be the sole basis for resolving the admissibility of expert testimony. Since 1993, most expert witness challenges arise from this holding.

In General Electric Co. v. Joiner (1997), the court held that the evidence that expert testimony relies upon must not contain gaps which would bring into question the veracity of those conclusions. It further held that appellate courts use an “abuse of discretion” standard in reviewing whether a trial court properly admitted testimony. These holdings augment Daubert, providing a more granular mechanism to analyze expert methodology.

In Kumho Tire Co. v. Carmichael (1999), the court held that all expert testimony (not just scientific) is subject to Daubert (and therefore Rule 702). Accountants, horse trainers, and astrophysicists are all held to the identical standard. While non-scientific experts have routinely been challenged under Daubert, this explicit holding settled the question.

Rule 702 in 2023

When taken together with the various amendments, there has been considerable variation to the application of Rule 702, leading to uncertainty for attorneys and experts alike in Federal Courts, but also in State Courts which have applied differing versions and interpretations of the rules. In order to remove some of the unpredictability from the process, a 2023 amendment clarified that the rules specifically affect admissibility, not weight. In other words, if a single prong of the analysis is lacking, the expert witness testimony should be excluded altogether. The Committee Note for the amendment explains that federal court holdings which consider Rule 702 a question of weight rather than admissibility “are an incorrect application of Rules 702 and 104(a)”.

This means that attorneys in Rule 702 Jurisdictions, even those who have adopted earlier revisions of the Rule, will be more careful to fully vet expert witnesses. In fact, several Federal cases have already been successfully appealed due to improperly admitting expert testimony under the amended rules.


Surviving a Daubert (or similar) challenge can be one of the more stressful aspects of expert witnessing, but careful vetting and good preparation can head them off. Once an expert has survived a challenge, it does not make it impossible to be challenged again in another case, and it cannot be used as direct evidence in future cases. However, it does bolster credibility and may provide a strategic advantage to experts, who can better prepare for the types of questions they are most likely to receive.

First, a motion to exclude an expert or specific testimony is filed. This is usually done pre-trial (in limine) but challenges can be made at any point in most jurisdictions, and even in limited post-trial situations. Next, a hearing takes place outside of the presence of the jury. Both sides will present arguments in a “mini-trial”, where experts may feel like defendants; accused of not being an expert, relying on flawed methodology, or deviating from standard practices per Rule 702 (or similar framework).

The expert’s proponent (your attorney) has the burden of proof and must show the expert most likely (the 50% standard) clears the hurdles of 702. Expert Witnesses will be cross-examined, proponents will redirect and rebut, and the judge will make an informed decision on the admissibility of either specific expert testimony, or all expert testimony.

Understanding the legal framework which governs the admissibility of expert testimony is crucial for expert witnesses. Rule 702 is a good starting point with the widest application, and even jurisdictions which use other rulesets have similar (or identical requirements). Your experience, knowledge, and credentials will serve as a bulwark against potential challenges, and a strong grasp of Rule 702 will allow you to effectively present your opinion in a way that maximizes the impact of your testimony.

If you are interested in expert witnessing, consider signing up with Round Table Group. For nearly 30 years, we have helped litigators locate, evaluate, and employ the best and most experienced expert witnesses. Contact us at 202-908-4500 for more information or sign up now!

  1. Capozzola, Damian D. Expert Witnesses in Civil Trials: Effective Preparation and Presentation.  2023rd–2024th ed., Thomson Reuters.


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