Expert opinions are pivotal in a wide variety of legal proceedings. They help judges and juries understand specialized topics like medical standards or patent law, as well as shape legal strategy. Often, they are a source of important knowledge when more rigorous data is not available or areas of study are too large for a thorough quantitative analysis.
However, expert opinion (EO) is often misused or misinterpreted by some. It is not a substitute for empirical data, and it should be used with caution to avoid bias in interpretations of the data or conclusions reached by the experts. Some societies have explicitly acknowledged the role of EO in developing practice guidelines. For example, the Centers for Disease Control and Prevention develops interim guidelines using a combination of EO or indirect and emerging evidence, which means that they might change when more research becomes available. Similarly, the European Society for Clinical Microbiology and Infectious Diseases notes that “Expert opinions might be used when there is no direct evidence, and should be provided with an explicit acknowledgment that they are based on expert opinion.”
Generally speaking, experts are expected to base their conclusions or inferences on facts or data commonly relied upon by professionals in the same field. If they receive information about a case that they have not previously personally observed, however, this fact or data should be clearly disclosed to the judge and jury during cross-examination. In some jurisdictions, the issue is resolved by resorting to Rule 705, which allows an expert to offer an opinion based on sources other than firsthand observation.
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