Federal Rule of Evidence 702 speaks in very general terms. It governs every situation in which "scientific, technical or other specialized knowledge will assist the trier of fact," and provides that, in that situation, "a witness qualified as an expert by knowledge, skill, experience, or education, may testify thereto in the form of an opinion or otherwise . . . .' In 2000, following a trio of Supreme Court cases interpreting Rule 702, the Rule was amended to include a third requirement, in addition to the helpfulness of the testimony and the qualifications of the witness: reliability. Under Rule 702 as amended, a qualified witness may only provide expert testimony "if (1) the testimony is based upon sufficient facts or data, (2) the testimony is the product of reliable principles and methods, and (3) the witness has applied the principles and methods reliably to the facts of the case. 2
Gross, Samuel R. "Expert Information and Expert Evidence: A Preliminary Taxonomy." J. L. Mnookin, co-author. Seton Hall L. Rev. 34, no. 1 (2003): 141-89.