Admissible evidence, in a court of law, is any testimonial, documentary, or tangible evidence that may be introduced to a factfinder—usually a judge or jury—in order to establish or to bolster a point put forth by a party to the proceeding. In order for evidence to be admissible, it must be relevant, without being prejudicial, and it must have some indicia of reliability.
For evidence to be relevant, it must tend to prove or disprove some fact that is at issue in the proceeding. However, if the utility of this evidence is outweighed by its tendency to cause the factfinder to disapprove of the party it is introduced against for some unrelated reason, it will not be admissible. Furthermore, certain public-policy considerations bar the admission of otherwise relevant evidence.
For evidence to be reliable enough to be admitted, the party proffering the evidence must be able to show that the source of the evidence makes it so. If the evidence is in the form of witness testimony, the party introducing the evidence must lay the groundwork for the credibility of the witness, and his knowledge of the things to which he attests. Hearsay is generally barred for its lack of reliability. If the evidence is documentary, the party proffering the evidence must be able to show that it is authentic, and must be able to demonstrate the chain of custody from the original author to the present holder. The trial judge performs a "gatekeeping" role in excluding unreliable testimony. The United States Supreme Court first addressed the reliability requirement for experts in the landmark case Daubert v. Merrell Dow Pharmaceuticals, Inc. 509 U.S. 579 (1993). The Court laid out four non-exclusive factors that trial courts may consider when evaluating scientific expert reliability: (1) whether scientific evidence has been tested and the methodology with which it has been tested; (2) whether the evidence has been subjected to peer review or publication; (3) whether a potential rate of error is known; and (4) whether the evidence is generally accepted in the scientific community. Id. at 592-94. Kumho Tire Co., Ltd. v. Carmichael later extended the Daubert analysis to include all expert testimony. 526 U.S. 137 (1999).