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 Oregon, the admissibility of expert witness testimony is governed by Oregon Revised Statutes and case law, which align with the principles of the Federal Rules of Evidence. An expert witness in Oregon is someone who has specialized knowledge, skill, experience, training, or education that can help the court understand the evidence or determine a fact in question. The decision to qualify a witness as an expert rests with the judge, who serves as a gatekeeper to ensure that the expert's testimony is relevant and reliable. The judge evaluates whether there is an appropriate fit between the witness's expertise and the subject matter of the case. This involves considering the methodology and principles underlying the expert's opinion to determine if they are scientifically valid and applicable to the facts of the case. The judge's role is to exclude expert testimony that is not based on a reliable foundation or is not relevant to the issues at hand, ensuring that only helpful expert testimony is presented to the jury or factfinder.