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 Massachusetts, the admissibility of expert witness testimony is governed by both state statutes and case law. An expert witness 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 judge, who serves as a gatekeeper to ensure that only reliable expert testimony is presented to the jury or judge. The judge must determine that there is an appropriate fit between the expert's expertise and the subject matter of the case. This involves assessing whether the expert's methods are reliable and whether their testimony is relevant to the issues at hand. Massachusetts follows the Daubert-Lanigan standard, which is a combination of the federal Daubert standard and the Commonwealth's own Lanigan decision, to evaluate the reliability and relevance of expert testimony. Under this standard, the judge evaluates factors such as the theory's testability, the known or potential rate of error, and general acceptance in the relevant scientific community.