In Federal Courts, the opinion testimony of an expert witness is not allowed to be introduced into evidence unless the experts opinions are based upon sound scientific methodology...it is not enough for an expert witness to simply base his or her opinion on their experience. This standard came from the Daubert case, which resulted in an amendment to Rule 702 of the Federal Rules of Evidence. By the way, many state courts have similar rules of evidence regarding expert testimony. Thus, a competent judge will not allow an expert to render an opinion that is based on nothing more than his or her experince with no else to support the opinion.
Rule 702. Testimony by Expert Witnesses
A witness who is qualified as an expert by knowledge, skill, experience, training, or education may testify in the form of an opinion or otherwise if the proponent demonstrates to the court that it is more likely than not that:
(a) the expert’s scientific, technical, or other specialized knowledge will help the trier of fact to understand the evidence or to determine a fact in issue;
(b) the testimony is based on sufficient facts or data;
(c) the testimony is the product of reliable principles and methods; and
(d) the expert's opinion reflects a reliable application of the principles and methods to the facts of the case.