Texas Rule of Evidence 702 governs the admissibility of expert witness testimony. That rule states: 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.
Kelly v. State is a 1992 case where the Texas Criminal Appeals court ruled that to be sufficiently reliable and relevant to help the jury, scientific evidence must meet three criteria: 1. The underlying scientific theory must be valid; 2. The technique applying the theory must be valid; and 3. The technique must have been properly applied on the occasion in question.
The proponent of the evidence must show the trial court in a preliminary hearing outside the jury's presence that the expert testimony meets all three criteria.