How Judges Decide Whether Expert Testimony Can Be Used at Trial

Update (October 2026): Federal Rule of Evidence 702 was amended effective December 1, 2023. The change makes clear that the party offering an expert must show it is "more likely than not" that the testimony meets each of the rule's requirements. See Fed. R. Evid. 702 and the 2023 committee note.

A federal judge may let jurors hear expert testimony only if the expert is qualified and the opinion is reliable and relevant. The judge acts as a "gatekeeper" because technical evidence can strongly sway a jury.

The Daubert standard and Rule 702

In Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993), the Supreme Court held that under Federal Rule of Evidence 702, the trial judge screens scientific evidence before the jury hears it. Later cases extended that gatekeeping role to other kinds of expert opinion as well. See Kumho Tire Co. v. Carmichael, 526 U.S. 137 (1999).

Under current Rule 702, a witness who is qualified as an expert by knowledge, skill, experience, training, or education may testify in the form of an opinion if the proponent shows it is more likely than not that:

1. the expert's scientific, technical, or other specialized knowledge will help the jury understand the evidence or decide a fact in issue;
2. the testimony is based on sufficient facts or data;
3. the testimony is the product of reliable principles and methods; and
4. the expert's opinion reflects a reliable application of those principles and methods to the facts of the case.

Daubert listed factors courts often use when judging reliability — whether the theory can be (and has been) tested, peer review and publication, known or potential error rate, standards controlling the technique, and general acceptance in the relevant field. Those factors are guides, not a checklist. The judge decides which ones matter in a given case.

How a challenge usually works

Defense lawyers often raise a Rule 702 / Daubert challenge in a motion in limine before trial, so the judge can hold a hearing outside the jury's presence. Under Federal Rule of Criminal Procedure 16(a)(1)(G), the government must, on request, give a written summary of each expert's opinions, the bases and reasons for them, and the expert's qualifications. Getting that disclosure early lets the defense hire a consulting expert, test the methods, and decide whether to move to exclude.

If the judge admits the testimony over objection, the issue can be raised on appeal. Even then, an appeals court may find the error "harmless" if other evidence of guilt was strong.

An Eleventh Circuit example: Barton

In United States v. Barton, Barton was convicted under 18 U.S.C. § 922(g) of being a felon in possession of a firearm. A government expert testified about DNA found on the gun and explained how one DNA sample is told apart from others. Barton argued the DNA methods were not reliable. He did not dispute the expert's qualifications or that the evidence was relevant.

The Eleventh Circuit rejected the challenge. It found that even without the DNA testimony there was strong evidence he had the gun — he admitted it when arrested and later to a jailhouse informant. So even if admitting the DNA opinion was a mistake, it was harmless.

Barton shows two practical points: reliability is the fight most often worth having, and even a strong Daubert objection may not win on appeal if the rest of the record supports the conviction.

What this means for you

Expert testimony on DNA, ballistics, drug quantities, accounting, cell-site location, or other technical subjects can decide a federal case. Challenge it early. Ask for the Rule 16 expert summary, dig into the methods, and bring in a defense expert when the science is central to the government's proof.

If the government plans to use an expert against you in federal court, Request a consultation so we can review the disclosure and decide whether a Rule 702 challenge makes sense.