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 Wisconsin, the admissibility of expert witness testimony is governed by state statutes and case law. An individual may qualify as an expert witness based on their knowledge, skill, experience, training, or education, and may testify if their expertise can help the court understand the evidence or resolve a disputed fact. However, there is no strict formula for determining who qualifies as an expert. The key requirement is that there must be a relevant connection between the witness's expertise and the subject matter of the case. The judge in the case serves as a gatekeeper, with the responsibility to ensure that only reliable expert testimony is presented to the judge or jury. This gatekeeping function involves assessing the relevance and reliability of the proposed expert testimony, which includes evaluating the methods, principles, and reasoning applied by the expert to ensure they are scientifically sound and applicable to the facts of the case.