Expert testimony

An expert witness, in England, Wales and the United States, is a person whose opinion by virtue of education, training, certification, skills or experience, is accepted by the judge as an expert. The judge may consider the witness's specialized (scientific, technical or other) opinion about evidence or about facts before the court within the expert's area of expertise, referred to as an "expert opinion". Expert witnesses may also deliver "expert evidence" within the area of their expertise. Their testimony may be rebutted by testimony from other experts or by other evidence or facts.

In Scots Law, Davie v Magistrates of Edinburgh (1953) provides authority that where a witness has particular knowledge or skills in an area being examined by the court, and has been called to court in order to elaborate on that area for the benefit of the court, that witness may give evidence of his/her opinion on that area.

Typically, experts are relied on for opinions on severity of injury, degree of sanity, cause of failure in a machine or other device, loss of earnings and associated benefits, care costs, and the like. In an intellectual property case an expert may be shown two music scores, book texts, or circuit boards and asked to ascertain their degree of similarity. In the majority of cases, the expert's personal relation to the defendant is considered and irrelevant.

The tribunal itself, or the judge, can in some systems call upon experts to technically evaluate a certain fact or action, in order to provide the court with a complete knowledge on the fact/action it is judging. The expertise has the legal value of an acquisition of data. The results of these experts are then compared to those by the experts of the parties.

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