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 New York, expert witnesses are permitted to testify in legal proceedings if their specialized knowledge, skill, experience, training, or education can help the court understand the evidence or resolve a disputed fact. 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 in relation to the subject matter of the case. The judge serves as a gatekeeper, assessing the appropriateness of the expert testimony to ensure that it is pertinent and reliable. If the judge finds the testimony to be unreliable or not applicable to the case at hand, it will be excluded from consideration. This gatekeeping role is crucial to maintaining the integrity of the legal process and ensuring that only credible and relevant expert testimony influences the outcomes of legal disputes.