Expert Testimony

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An opinion stated during a trial or deposition (testimony under oath before trial) by an expert witness on a subject relevant to a lawsuit or a criminal case. The subject will usually be technical or scientific, such as ballistics, forensics, or medical. The Federal Rules of Evidence, Rule 702 sets out the following requirements for expert testimony to be admissible, however. (a) that 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) that “the testimony is based on sufficient facts or data;” (c) that “the testimony is the product of reliable principles and methods;” and (d) that “the expert has reliably applied the principles and methods to the facts of the case.” For example, in Kumho Tire Co. v. Carmichael, the Supreme Court affirmed a District Court’s decision not to admit expert testimony on tire marks and treads because there was “no indication in the record that other experts in the industry use [the expert]’s particular approach or that tire experts normally make the very fine distinctions necessary to support his conclusions.”

[Last updated in November of 2020 by the Wex Definitions Team]