Reliability and Validity of Forensic Science Evidence

Abstract

The rules of evidence both authorize and limit testimony from scientific, medical and other experts. For example, Federal Rule of Evidence 702 allows experts to supply information or opinions to the jury only when “the testimony is . . . the product of reliable principles and methods … reliably applied … to the facts of the case”. The law's use of the word “reliably” to connote probable factual accuracy is potentially confusing, since “reliability” has a distinct meaning in statistical science. The law is concerned with both “reliability” in the statistical sense and “validity” of forensic science methods. This article describes how scientists define and assess the reliability and validity of some commonly encountered types of forensic science evidence. Such assessments are necessary for courts to admit putatively scientific evidence as bona fide and legally “reliable” science. The essay is part of a special issue of Significance, a journal of the Royal Statistical Society and the American Statistical Association.

Keywords: forensic science, scientific evidence, reliability, validity, Daubert, Rule 702