An expert opinion is a specialized conclusion or inference drawn by a person who has trained and/or experienced in a particular field. Such opinions are important to legal cases because they help a judge or jury understand complex issues that might not be readily understood without the expertise of an expert. An expert’s opinion must be based on reliable methods and data commonly relied upon by professionals in that field. Otherwise, such opinions may be excluded from evidence because they are unreliable.
Many experts are retained by parties to express their opinions in court for the benefit of the parties and the trier of fact. Speculation and guessing are not allowed. The process by which an expert develops his or her opinions must follow a rigorous course. The expert must be prepared to defend the facts and information on which his or her opinion is based against challenge during cross-examination.
Expert witnesses are often asked to provide opinions on topics such as severity of injury, degree of sanity, cause of a machine or device failure, loss of earnings and associated benefits, damages for pain and suffering, and the like. The reliability of an expert’s opinion depends on several factors, including (1) whether the expert is recognized by peers in the field to have specialized knowledge; (2) whether the methodology used to form the opinion can be tested and has been; (3) whether the method is widely accepted within a relevant scientific community; and (4) whether the expert has a conflict of interest that renders him or her unreliable.