An expert opinion is a conclusion or inference drawn by a qualified individual in his or her field of expertise. It is based on the expert’s specialized education, training and experience, and is intended to assist the fact-finder in understanding complex issues in the case. Expert opinions are generally based on reliable methods, instruments and data that are established and relied upon by professionals in the expert’s peer community.

The use of expert opinions in arguments can help to enhance the credibility and weight of an argument, making it more persuasive. However, the expert’s qualifications must be carefully evaluated, as well as the accuracy of the information he or she is using. A thorough investigation of the expert’s credentials may include verification of membership requirements, a review of professional publications and his or her reputation in the community.

Experts may be called upon to testify in court to support claims or counterclaims made in a lawsuit. Such testimony may be in the form of a written report or in person during trial. Regardless of the format, expert opinions must be thoroughly examined by opposing counsel in cross-examination to determine whether the conclusions are supported by reliable evidence or merely anecdotal observations.

Among the 65 societies that mention EO, terminology varies widely (Additional file 3). The most common term is “expert opinion” (32%), followed by terms highlighting consensus processes (29%), such as “consensus-based recommendation” or “best practice guidelines”. Other commonly used terms include “expert judgment”, “clinical judgment” and “informed perspective”. Several societies are unclear about how to describe their use of EO and are not able to provide details in their manuals.