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Expert Witness (Daubert and Frye)

A witness allowed to give opinions rather than only facts because of specialised knowledge - who qualifies, the gatekeeping test a judge applies to the reliability of the method (Daubert in federal court and most states, Frye in the rest), the written report and deposition that precede trial, the difference between a retained expert and a treating professional, and why a case can be lost when its expert is excluded.

Informational only - this is not legal advice. These definitions explain general legal vocabulary in plain English. They are not advice about your situation, reading them creates no attorney-client relationship, and the law differs from state to state and changes over time. For advice you can rely on, speak to a lawyer licensed in your state.

What it means

Ordinary witnesses testify to what they saw, heard or did. An expert witness is permitted to do something no other witness may: give an opinion, on a matter beyond the understanding of an ordinary juror, based on scientific, technical or other specialised knowledge. Experts appear in most serious civil cases and many criminal ones - a physician on the cause of an injury and the standard of care, an engineer on why a product failed, an accountant on lost profits, an economist on damages, a forensic scientist on DNA or a firearm, an appraiser on value - and in professional-negligence cases the law usually requires one, because the standard of care itself has to be proved by someone in the field. A witness qualifies by knowledge, skill, experience, training or education; a formal credential is not required, and a credential does not qualify someone to opine outside their real expertise.

The court acts as gatekeeper. Before an expert may testify, the judge must be satisfied that the testimony rests on sufficient facts or data, that it is the product of reliable principles and methods, and that the expert has reliably applied those methods to the facts of the case; the party offering the expert bears the burden of showing this, usually on a pretrial motion to exclude that is decided on the report, the deposition and sometimes a hearing. In federal court and in most states the test is the Daubert standard, under which the judge considers such things as whether the method has been tested, subjected to peer review, has a known error rate and standards controlling its use, and is generally accepted in the relevant community - factors, not a checklist, applied to every kind of expertise, not only laboratory science. A minority of states keep the older Frye test, which asks only whether the technique is generally accepted in its field. Under either test the question is the reliability of the method, not whether the conclusion is right; an admitted expert is then cross-examined and answered by the other side's expert, and the jury decides whom to believe.

Expert work is governed by disclosure rules. A retained expert must be identified by a deadline in the scheduling order and must produce a signed written report stating every opinion, its basis and reasons, the data considered, the expert's qualifications and publications, prior testimony and compensation; an expert who has not been properly disclosed may be barred, which can end a claim that depends on expert proof. The expert is then deposed. Communications between the lawyer and a retained expert, and the expert's drafts, are largely protected, but the facts and assumptions the lawyer supplied are discoverable. A treating physician or other professional who formed opinions in the course of their work may testify to those opinions on a lighter disclosure without a full report. A consulting expert who will not testify is ordinarily undiscoverable at all. Experts are paid for their time by the party who retains them, may not be paid on a contingency, and their fees are part of the cost of litigation that a client should expect to bear, since they are rarely recoverable from the loser.

Where this comes from

In federal court the admissibility rule is Federal Rule of Evidence 702, amended in 2023 to make explicit that the proponent must establish each requirement by a preponderance; the gatekeeping trilogy is Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993), General Electric Co. v. Joiner, 522 U.S. 136 (1997), and Kumho Tire Co. v. Carmichael, 526 U.S. 137 (1999). The older test is Frye v. United States, 293 F. 1013 (D.C. Cir. 1923), retained by a minority of states including New York and, in modified form, California under People v. Kelly, 17 Cal. 3d 24 (1976), and Sargon Enterprises, Inc. v. University of Southern California, 55 Cal. 4th 747 (2012). The basis of opinion and its disclosure are Federal Rules of Evidence 703 and 705, and the disclosure, report and deposition rules are Federal Rule of Civil Procedure 26(a)(2) and 26(b)(4), including the work-product protection for drafts and lawyer-expert communications at 26(b)(4)(B)-(C); exclusion for non-disclosure is Rule 37(c)(1). Each state's evidence code and civil rules govern its own courts, and the disclosure deadlines are set by the scheduling order in each case.

When people hire a lawyer for this

The choice of expert, and the timing of the retention, is one of the decisions on which cases turn: an expert retained late may not have time to do the work a report requires, and an expert whose method does not survive a Daubert motion takes the claim down with them. Anyone bringing a medical-malpractice, product, construction or professional-negligence case should ask at the first meeting what expert proof the claim needs and what it will cost, since the expense is real and is usually advanced by the client or the firm. Professionals asked to serve as experts should understand that a report is a sworn statement of every opinion they will give, that everything they relied on is discoverable, and that their prior testimony will be searched for inconsistency.

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Part of the LawyerLand plain-English legal glossary. Definitions are written from primary sources - statutes and court rules - and each entry states the authority it rests on, or says plainly when the doctrine is state law with no national rule.

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