Normally, witnesses in a trial are limited to testifying to what they have perceived. That is, what they saw, smelled, tasted, heard, etc. An expert witness is a special kind of witness. An expert witness can offer an expert opinion when testifying. Expert witness testimony is governed by Federal Rule of Evidence 702, which states:
Rule 702. Testimony by Expert Witnesses
A witness who is qualified as an expert by knowledge, skill, experience, training, or education may testify in the form of an opinion or otherwise if:
(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 has reliably applied the principles and methods to the facts of the case. (emphasis added).
Expert opinions are given in the vast majority of trials. The topics on which expert opinions are provided vary greatly and depend upon the issues at hand in the case. Expert opinions are commonly given on issues such as DNA, valuation, medicine, engineering, accident reconstruction, fingerprints, and countless others. To get a sense of the diverse range of expert opinions being offered, you can browse the expert witness specialties and profiles in SEAK’s National Expert Witness Directory.