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 Washington State, expert witnesses are permitted to testify in legal proceedings if their specialized knowledge, skills, experience, training, or education can help the court understand the evidence or decide on a matter in dispute. The state follows the guidelines similar to the federal Daubert standard, which requires that the testimony is based on sufficient facts or data, is the product of reliable principles and methods, and that the expert has applied the principles and methods reliably to the facts of the case. 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 and reliability of the expert testimony before it is presented to the jury. This ensures that only pertinent and trustworthy expert evidence is considered in the courtroom.