For years, the admissibility of expert scientific evidence was subject to the Frye test, based on Frye v. United States (1923). Under Frye, only expert scientific evidence based on "generally accepted" principles in the scientific community was admissible. In 1973, the Federal Rules of Evidence were adopted. The Federal Rule of Evidence 702, however, stated the following, in contrast to Frye:
If scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education, may testify thereto in the form of an opinion or otherwise.
The Supreme Court determined in Daubert whether the Federal Rules of Evidence superseded the Frye test.
Daubert was a product liability action in which the plaintiffs, in essence, sought to establish that the ingestion of a prescription drug caused birth defects. The Court limited its analysis to "scientific" knowledge. The Court found that expert testimony must possess scientific validity to establish evidentiary reliability. Further, the Court found that "all relevant evidence is admissible" and that relevant evidence must "assist the trier of fact to understand the evidence or determine a fact in issue."
While the Court did not adopt "a definitive checklist or test" to determine the reliability of expert scientific testimony, it articulated four important factors:
whether the theory or techniques can be (and has been) tested;
whether the techniques or theory has been subjected to peer review and publication;
whether the techniques employed by the expert have a known or potential rate of error and standards controlling the technique's operations; and
whether the theory or technique employed by the expert has been generally accepted by the scientific community.
It is important to note that Daubert emphasizes that testimony may be admissible even where one or more of these factors are unsatisfied. For example, publication "is not a sine qua non [essential condition] of admissibility," and "does not necessarily correlate with reliability," according to the Court. Daubert emphasizes, however, that two criteria must be met: (1) the evidence must be relevant, as required under Rule 702; and (2) the testimony must be "derived by the scientific method" and "supported by appropriate validation."