Law 101 for Experts: Understanding the Laws That Govern Expert Witness Testimony

Generally speaking, court rules prohibit a witness from testifying to their own opinions or analyses. Witnesses are expected to stick to the facts of what they saw, heard, or otherwise experienced. 

Dani Alexis Ryskamp, J.D.

Written by
— Updated on June 22, 2022

Law 101 for Experts: Understanding the Laws That Govern Expert Witness Testimony

These court rules change when the witness is certified as an expert witness, however. Expert witnesses can testify about matters that lie in their field of expertise. This includes their own opinions and analyses formed with the reasoning and tools experts apply in that field. Expert witness testimony strays into the realm of opinion and analysis. As such, the bar for qualifying as an expert witness is higher than that for a fact or lay witness.

Understanding the laws that govern expert witness testimony can help experts clarify their approach. It can also help experts prepare for challenges and provide the context and insight for attorneys who retained their services. 

Standards and Requirements for Expert Witness Testimony

Each state has its own state court rules governing expert witness testimony. Many, but not all, states pattern these rules on the Federal Rules of Civil Procedure (FRCP).

In federal courts, Article VII of the FRCP lays out the ground rules for opinion testimony, including expert witness testimony.

The Federal Rules of Civil Procedure provide a framework, but the rules themselves leave many questions unanswered. Various U.S. Supreme Court cases clarified some matters, including Daubert v. Merrell-Dow Pharmaceuticals (1993).

Daubert Standard

Daubert articulated a standard for evaluating scientific or technical knowledge admitted as evidence in a court case. The court held that when such evidence was at issue, the trial court judge must determine whether an expert witness’s testimony is based on scientifically valid reasoning. The court must also determine whether the expert properly applied that reasoning to the facts at issue in the case.

Daubert articulated five factors for evaluating an expert witness’s methodology:

  • Whether the theory or technique in question can be and has been tested
  • Whether the theory or technique has been subjected to peer review and publication
  • The theory or technique’s known or potential error rate
  • Whether standards exist controlling the theory or technique’s application or operation, and if so, how those standards are maintained
  • Whether the theory or technique has attracted widespread acceptance within a relevant scientific community

U.S. federal courts later incorporated the Daubert standard into Federal Rule of Evidence 702. Daubert hearings are pretrial hearings that determine whether an expert’s work meets this standard.

The Daubert standard replaced a previous standard used in federal courts known as the Frye standard. In Frye v. United States (1923), the D.C. Circuit held that the other experts in the field must generally accept the methods an expert witness uses. While federal courts no longer apply the Frye standard, Frye remains in use in some U.S. state courts.

Not all theories, techniques, and other methods an expert witness may use are sufficiently controversial to draw a challenge from opposing counsel. Washing hands in order to reduce the spread of germs, for example, is so well understood that it is unlikely to be questioned under either the Daubert or Frye standard.

Not all methods are so well-received, however. One way opposing counsel may seek to prevent an expert from testifying is to file a motion seeking to exclude all or part of an expert witness’s opinion because the basis of that opinion doesn’t meet the requirements of FRE 702. This motion is a Daubert motion. (In states that still adhere to the Frye rule, the motion may be called a Frye motion and seek to apply the “generally accepted” standard.)

A Daubert motion may call into question an expert’s opinion based on any or all of the Daubert factors. Expert witnesses who seek to defend their work against a Daubert motion must be able to articulate the basis and validity of their methods, as well as the results. 

Rules Governing Expert Witness Testimony and Opinions

Unlike a lay witness, an expert witness may testify to their opinions on issues within their field of expertise. Yet experts may not pronounce any opinion they like.

Federal Rule of Evidence 703 states that an expert may state an opinion that is based “on facts or data in the case that the expert has been made aware of or personally observed.”

First, the opinion must be based on facts or data. It is therefore vital to record facts or data collected during the process. By tracking facts and data, experts can articulate how they then analyzed that information.

Second, the opinion must be based on facts or data that the expert is aware of or has personally observed.

The “personal observation” rule is similar to the rule for lay witnesses. Both expert and lay witnesses may testify to facts or data they personally saw, heard, or otherwise perceived. For example, an expert applied a color-changing reagent during a lab test. The expert may testify as to whether or not they saw the reagent change color during the test.

Experts also have leeway to rely on facts or data they “were aware of” in order to form their expert opinions. An expert in the properties of materials, for instance, doesn’t have to personally witness a ladder collapse in order to include the fact “the ladder collapsed” in their analysis.


Testimony about information received secondhand is often classified as hearsay. Unless an exception to the hearsay rule exists, such testimony is not admissible in court. Under FRE 703, however, an expert may still testify to an opinion based on those facts, even if the facts themselves are not admissible.

Opinions Experts Should (and Should Not) Offer

Expert witnesses are most likely to survive a Daubert challenge and provide optimal assistance in a case if their opinions:

  • Are based on longstanding principles applicable in their field
  • Focus on the facts and data available
  • Clearly indicate where information is missing that might affect the outcome of their analysis

Focusing on these elements helps experts avoid expressing their opinions in unhelpful or inadmissible ways. For example:

  • Avoid speculating about “what would have happened if…”
  • Resist the temptation to apply new, untested, or emerging techniques
  • Remember that the work is about your professional opinion, not your personal opinion

A well-formed expert opinion helps the parties clarify key issues. It also helps the court understand the context of the case in order to reach a more informed result.

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