While many jurisdictions do not require an expert to list all the facts or data he or she received before forming an opinion, experts will typically be expected to list these if asked to do so on cross-examination. Like lay witnesses, experts are not generally allowed to speculate. The resulting opinion must typically be based on available facts. In fact, this information is often essential: without it, the expert may be able to form no opinion at all. When an expert presents certain facts, it is important to establish how those facts support the opinion the expert expresses.Įxperts can receive information about the case on which to base their opinion.

For example, an expert is generally expected to base an opinion or inference on facts or data that are reasonably relied upon by similar experts in the subject area. For instance, the appropriate expert may discuss how a car accident occurred even though the expert was not at the scene when the crash took place. Experts can discuss the where, when, what, why, and how of events they did not personally witness. Unlike lay witness opinions, experts may range into matters outside what they experienced or can derive from their experience. Here, we explore the scope and limitations of expert opinions, including what experts may testify to, on what information they can base their testimony, how confident experts need to be in the “correctness” of their opinions, and the types of cases in which experts most commonly appear.

Expert witness services definition