When properly presented, expert testimony can be a powerful component of a case. However, it must be limited to matters within the scope of the expert’s expertise and avoid transgressing certain boundaries. This article explores those boundaries by reviewing the criteria that judges use to judge the admissibility of expert opinions, on what basis an expert may testify and how confident experts must be in the correctness of their opinions.
The foundation for the expert’s opinion must be valid and reliable, a principle derived from a series of court decisions known as Daubert. The trial court acts as the gatekeeper charged with excluding expert testimony that is unreliable, and a judge’s role in this process can have a significant impact on an expert’s reputation (Brautbar, 1999). A key factor in evaluating whether an expert’s theory or methodology meets the Daubert standard is whether it has been tested or replicated by others in the relevant scientific community. This can be done by examining an expert’s curriculum vitae and, in particular, looking for articles in journals that are renowned in the field.
Additionally, if the expert’s analysis is based on an extrapolation from a generally accepted hypothesis to a new and unsubstantiated conclusion, this can also be challenged under Daubert. A trial judge may ask the expert to address this by requesting him to present evidence that shows the hypothesis and its conclusions are based on data that is reasonably relied upon in the field. This can be accomplished in several ways, including through the familiar hypothetical question or through presenting the expert’s research materials at the trial.
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