What qualifies someone as an expert witness?
The qualifications for someone to be an expert witness varies from state to state. Each state has their own Rules of Evidence and cases that address the qualifications of a witness to testify in the form of an expert opinion. Typically, it involves scientific, technical or other specialized knowledge that would assist (or sometimes "substantially assist") the trier of fact (judge or jury) and the witness must be qualified as an expert based on their knowledge, skill, experience, training or education. (See Rules 702 of the Tennessee Rules of Evidence and Rule 702 of the Federal Rules of Evidence). Their opinions can be challenged, or even excluded, if they are unreliable or would not "assist the trier of fact" Non-exclusive factors for weighing the credibility of the expert evidence or testimony include: (1) whether the evidence has been tested and the methodology has been tested; (2) whether the evidence has been subjected to peer review or publication; (3) whether a potential rate of error is known; (4) whether the evidence is generally accepted in its industry or field; and (5) whether the expert's research in the field has been conducted independent of litigation. There are other factors, too numerous to list, that could come into play depending on the case or the types of opinions solicited.
Whether or not an expert is qualified to testify is in the sound discretion of the trial court after considering these factors. But the introduction of an expert's opinion is not determinative of an issue, or the ultimate issue, in a case. A judge or jury can give that expert's opinion whatever weight it feels is appropriate--even completely discounting an opinion. I've won several cases where the opposing side had an expert, and I did not, just by showing the jury that the opinion of the expert was contrary to common sense and experience.