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 Louisiana, expert witness testimony is governed by the Louisiana Code of Evidence, particularly Article 702, which aligns with the principles described. An expert witness in Louisiana may be qualified based on knowledge, skill, experience, training, or education, and may testify if their specialized knowledge will help the court understand the evidence or determine a fact in issue. 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 dispute. The trial judge serves as a gatekeeper, assessing the admissibility of expert testimony to ensure that it is pertinent and methodologically sound. The judge must find that the testimony is based on sufficient facts or data, is the product of reliable principles and methods, and that the expert has reliably applied the principles and methods to the facts of the case. This gatekeeping role is crucial to prevent unreliable expert testimony from being presented to the jury.