Expert Testimony

• Author: Jeffrey Lehman, Shirelle Phelps

Page 283

Testimony about a scientific, technical, or professional issue given by a person qualified to testify because of familiarity with the subject or special training in the field.

Generally speaking, the law of evidence in both civil and criminal cases confines the testimony of witnesses to statements of concrete facts within their own observation, knowledge, and recollection. Testimony must normally state facts perceived by the witnesses' use of their own senses, as distinguished from their opinions, inferences, impressions, and conclusions drawn from the facts. Opinion testimony that is based on facts is usually considered incompetent and inadmissible, if the factfinders are as well qualified as the witness to draw conclusions from the facts.

In certain instances, however, the law allows witnesses to provide opinion evidence, and such evidence is divided into two classes, lay opinion and expert opinion. A lay witness may give his or her opinion when that opinion is (1) rationally based on the perception of the witness; (2) helpful to a clear understanding of the witness' testimony or the determination of a fact in issue; and (3) not based on scientific, technical, or other specialized knowledge within the scope of expert testimony discussed below. Thus, lay witnesses who have had an opportunity to observe a particular vehicle in motion are normally permitted to testify that it was traveling at a great rate of speed or was going pretty fast. Lay witnesses are also normally allowed to give their opinion as to the height, weight, quantity, and dimensions of things, even if their testimony is not precise. By definition, a lay witness is any witness who is not qualified to testify as an expert on a particular subject.

Expert witnesses are persons who are qualified, either by actual experience or by careful study, to form definite opinions with respect to a division of science, a branch of art, or a department of trade. The law deems persons having no such experience or training to be incapable of forming accurate opinions or drawing correct conclusions. Thus, if scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education, may testify in the form of an opinion or otherwise, if (1) the testimony is based upon sufficient facts or data; (2) the...