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.

In Massachusetts, the admissibility of expert witness testimony is governed by both state statutes and case law, which align with the principles outlined in the Federal Rules of Evidence. An expert witness in Massachusetts is someone who has specialized knowledge, skill, experience, training, or education that can help the court understand evidence or determine a fact in question. The decision to qualify a witness as an expert rests with the trial judge, who serves as a gatekeeper to ensure that the expert's testimony is relevant and reliable. The judge evaluates whether there is an appropriate fit between the expert's expertise and the subject matter at issue in the case. This evaluation is not based on a strict formula but rather on the judge's discretion, guided by factors such as the expert's qualifications, the methodology used, and the applicability of the expert's reasoning to the facts of the case. The goal is to allow only expert testimony that is based on a reliable foundation and is relevant to the issues before the court, thereby aiding the judge or jury in their deliberations.