A witness who qualifies as an expert because of knowledge, skill, experience, training, or education may testify as an expert if scientific, technical, or other specialized knowledge will assist the judge, jury, or other factfinder to understand the evidence or determine a contested fact issue.

But there is no rigid formula for determining whether a particular witness is qualified to testify as an expert. Generally there must be a fit between the subject matter at issue and the expert witness’s familiarity with the subject matter. And this determination is made by the judge, who acts as a gatekeeper to only allow reliable expert witness testimony to be heard and considered by the judge or jury—and to exclude such testimony when it is not reliable.

In Delaware, the admissibility of expert witness testimony is governed by Delaware Rule of Evidence 702. This rule allows a witness who has knowledge, skill, experience, training, or education in a particular field to testify as an expert if their specialized knowledge will help the court understand the evidence or decide a fact in question. The determination of whether a witness qualifies as an expert is not based on a strict formula but rather on the relevance and reliability of the expert's knowledge to the subject matter of the case. The judge serves as a gatekeeper, ensuring that only reliable expert testimony is presented to the jury or considered by the judge. The judge evaluates factors such as the expert's qualifications, the methodology used, and the applicability of the expert's reasoning to the facts of the case to decide whether the testimony is admissible. This gatekeeping role is crucial to prevent unreliable or irrelevant expert testimony from influencing the outcome of a trial.