Expert testimony and Rule 702

A witness 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.

For decades the dominant test for novel scientific evidence was “general acceptance” in the field, from Frye v. United States (1923). The Supreme Court held that Frye’s general acceptance test was superseded by the adoption of the Federal Rules of Evidence.

Under the Rules, the trial judge must ensure “that an expert’s testimony both rests on a reliable foundation and is relevant to the task at hand.” Considerations include whether the theory or technique can be (and has been) tested, whether it has been subjected to peer review and publication, its known or potential error rate, the existence and maintenance of standards controlling its operation, and whether it has attracted widespread acceptance within a relevant scientific community.

The Court called the inquiry “a flexible one,” and said its focus “must be solely on principles and methodology, not on the conclusions that they generate.” It also said: “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.”

The plaintiffs’ case rested in significant part on a tire failure analyst who intended to testify that a defect caused a blowout. His opinion was based on a visual and tactile inspection of the tire.

The Supreme Court held that the gatekeeping obligation is not limited to “scientific” testimony: “it applies to all expert testimony.” A trial judge may consider one or more of the Daubert factors, but they do not constitute a definitive checklist or test. Whether they are pertinent depends on the nature of the issue, the expert’s particular expertise, and the subject of the testimony.

The objective, the Court said, is to make certain that an expert, “whether basing testimony upon professional studies or personal experience, employs in the courtroom the same level of intellectual rigor that characterizes the practice of an expert in the relevant field.”

The amendment made two changes. First, it states in the rule text that the proponent must show the requirements are met more likely than not. The committee note explains that this is the Rule 104(a) preponderance standard, and that “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).” Once the court finds a requirement met, further attacks go to weight.

Second, subsection (d) now says the opinion must reflect a reliable application of the method. The note says each opinion “must stay within the bounds of what can be concluded from a reliable application of the expert’s basis and methodology.”

The committee note says the amendment “is especially pertinent to the testimony of forensic experts in both criminal and civil cases.” It makes three points:

• Forensic experts should avoid assertions of absolute or one hundred percent certainty, or to a reasonable degree of scientific certainty, if the methodology is subjective and thus potentially subject to error.
• The judge should, where possible, receive an estimate of the known or potential rate of error of the method, based where appropriate on studies of how often it produces accurate results.
• Opinion on the weight of feature comparison evidence must be limited to the inferences that can reasonably be drawn from a reliable application of the principles and methods.

The note also says the amendment does not bar testimony that comports with substantive law requiring opinions to a particular degree of certainty.

Most investigators testify about what they saw and did. Rule 702 reaches them when an opinion rests on specialized knowledge. Under Rule 701, a lay opinion may not be “based on scientific, technical, or other specialized knowledge within the scope of Rule 702.”

The 2000 committee note to Rule 701 explains that the line falls between expert and lay testimony, not expert and lay witnesses. The same witness can give both in one case. It cites a Ninth Circuit example: agents could say a defendant was acting suspiciously without qualifying as experts, but the expert rules applied when, based on extensive experience, they testified that the defendant was using code words for drug quantities and prices.

Rule 702 and Daubert govern federal courts. Many states follow Daubert or a rule modeled on it, and some still use the Frye general acceptance test. Confirm the standard in the court where you will testify.

• Bench notes, validation records, and proficiency results are the material a proponent uses to show reliability. Keep them complete.
• Record what was examined, the method used, and how it was applied. Subsection (d) asks about this case, not the method in general.
• Know your method’s error rate and limits, and be ready to state them.
• Phrase conclusions within what the method supports. Avoid claims of absolute certainty for subjective methods.
• Investigators giving opinions from training and experience should expect those opinions to be treated as expert testimony.

• Federal Rule of Evidence 702, with committee notes
• Federal Rule of Evidence 701, with committee notes
• Supreme Court order amending the Federal Rules of Evidence, April 24, 2023
• United States Courts, Federal Rules of Evidence
• Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993)
• Kumho Tire Co. v. Carmichael, 526 U.S. 137 (1999)