Expert opinion is an inference based on specialized knowledge, training or experience. It is a tool that can be used to help fact-finders understand complicated issues in a case. However, experts can expect to be closely questioned about the facts and data on which they are relying when expressing their opinions. If the basis for an expert’s opinion is found to be inadequate, it may be excluded from the trial by a judge.
The information on which an expert’s opinion is based can come from three sources: firsthand observation (as in the case of a treating physician), research in the expert’s field, and reports submitted to regulatory agencies such as the U.S. Food and Drug Administration. Each of these sources can contain useful information, but it is important that the expert clearly state in his or her testimony which type of information is being relied upon.
In general, an expert’s evidence must satisfy Federal Rule 702 and the landmark court decisions Daubert and their progeny. These factors, which can be summed up as whether the expert’s theory or methodology can be tested; whether it has been published; whether there is known error rate for the methods; and whether the method is widely accepted within a relevant scientific community, will serve as screening tools to weed out unreliable evidence. The reliance on such unreliable information can severely harm a plaintiff’s case and can lead to appeals or mistrials in some cases.