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 Missouri, expert witnesses are permitted to testify in legal proceedings if their specialized knowledge, skills, experience, training, or education can help the judge or jury understand the evidence or decide on a contested fact. The state does not adhere to a strict formula for qualifying an expert witness. Instead, the key consideration is the relevance and fit between the expert's expertise and the subject matter of the case. The judge serves as a gatekeeper, assessing the reliability and relevance of the expert testimony before it is presented to the jury. This gatekeeping function is crucial to ensure that only pertinent and reliable expert testimony is considered in the courtroom. The judge's decision on whether to admit expert testimony is guided by factors such as the methodology used by the expert, the expert's professional background, and the applicability of the expert's knowledge to the case at hand.