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 Washington State, the admissibility of expert witness testimony is governed by the rules of evidence, specifically Rule 702 of the Washington Rules of Evidence. This rule allows a witness who has knowledge, skill, experience, training, or education in a particular field to testify as an expert 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 whether there is an appropriate fit between the witness's expertise and the subject matter of the dispute. The trial judge serves as a gatekeeper, assessing the reliability of the expert's testimony and ensuring that only relevant and reliable expert evidence is presented to the jury. The judge has the discretion to exclude expert testimony that does not meet the standards of reliability and relevance set forth in the legal framework.