Expert Testimony

Introduction

Expert testimony plays a central role in modern litigation, enabling parties to present specialized knowledge to assist the trier of fact. The admissibility of expert testimony is governed by FRE 702 and the framework established by the Supreme Court in Daubert v. Merrell Dow Pharmaceuticals, Inc. (1993). The Daubert standard requires trial judges to act as gatekeepers, ensuring that expert testimony is both relevant and reliable before it is presented to the jury.

The Frye Standard

Before Daubert, the dominant standard for expert testimony was the Frye test, established in Frye v. United States (1923). Under Frye, scientific evidence was admissible only if the scientific principle on which it was based had gained general acceptance in the relevant scientific community. The test focused on the consensus within the scientific community rather than on the reliability of the particular methodology used in the case.

Several states continue to follow Frye, including California, Illinois, and New York. The Frye standard is generally considered more lenient than Daubert in admitting expert testimony, as it does not require the trial judge to independently evaluate the reliability of the methodology.

The Daubert Standard

In Daubert v. Merrell Dow Pharmaceuticals, Inc., the Supreme Court held that FRE 702 supersedes Frye and requires trial judges to perform a gatekeeping role to ensure that expert testimony is both relevant and reliable. The Court identified several nonexclusive factors for evaluating reliability: whether the theory or technique can be (and has been) tested; whether it has been subjected to peer review and publication; the known or potential rate of error; the existence and maintenance of standards controlling the technique’s operation; and the degree of acceptance within the relevant scientific community.

General Electric Co. v. Joiner (1997) held that appellate review of Daubert decisions is for abuse of discretion. Kumho Tire Co. v. Carmichael (1999) held that the Daubert gatekeeping obligation applies to all expert testimony, not just scientific testimony. The trial judge’s gatekeeping role extends to testimony based on technical or other specialized knowledge.

FRE 702 and the 2023 Amendment

FRE 702 codified the Daubert standard and was amended in 2023 to clarify that the proponent of expert testimony must demonstrate the admissibility requirements by a preponderance of the evidence. The amended rule requires that: the expert’s knowledge, skill, experience, training, or education helps the trier of fact; the testimony is based on sufficient facts or data; the testimony is the product of reliable principles and methods; and the expert has reliably applied the principles and methods to the facts of the case.

The 2023 amendment rejected the practice of admitting expert testimony subject to later challenges to its reliability. The amendment requires the court to find that the expert’s opinion is more likely than not reliable before it may be admitted.

Reliability Gatekeeping

The trial court’s gatekeeping function requires a rigorous examination of the expert’s methodology. The court must examine whether the expert’s reasoning and methodology are scientifically valid and can be applied to the facts at issue. The court may consider whether the expert has unjustifiably extrapolated from an accepted premise to an unfounded conclusion, whether the expert adequately accounted for alternative explanations, and whether the expert’s opinion was developed for litigation or from independent research.

Reliability does not require that the expert’s opinion be correct or that it be generally accepted. The standard is whether the methodology is reliable, not whether the conclusion is convincing. Disputes about the weight of the evidence are for the jury to resolve once the threshold reliability requirement is satisfied.

Lay Versus Expert Opinion

FRE 701 permits lay witnesses to offer opinion testimony that is rationally based on their perception and helpful to understanding the testimony or determining a fact. Lay opinion is limited to opinions that are not based on scientific, technical, or other specialized knowledge. The distinction between lay and expert opinion is important because expert testimony must satisfy the Daubert requirements.

Conclusion

Expert testimony is essential in many cases, and the admissibility framework under Daubert and FRE 702 ensures that only reliable expert evidence reaches the jury. The trial judge’s gatekeeping role, the factors for evaluating reliability, and the distinction between lay and expert opinions define the boundaries of admissible expert testimony. The 2023 amendment to FRE 702 reinforces the requirement that the proponent demonstrate reliability by a preponderance of the evidence before admission.