The Effect of Uncertainty About the Alternative Source Population on the Assessment of the Value of Forensic Evidence

• Activity Number: 524
• Type: Contributed
• Date/Time: Wednesday, August 1, 2012 : 10:30 AM to 12:20 PM
• Sponsor: IMS
• Author(s): Christopher Saunders*+
• Companies: George Mason University
• Keywords: Forensics ; Bayes Factors

Abstract

In the forensic interpretation of scientific evidence is to make an inference about the source of a trace of unknown origin. This inference usually concerns two propositions: the first (usually referred to as the prosecution hypothesis) states that a given specific source is the actual source of the trace of unknown origin, and the second (usually referred to as the defense hypothesis) states that the actual source of the trace of unknown origin is randomly selected from a relevant alternative source population. The evidence a forensic scientist is given for deciding between these two propositions is: (a) the trace of unknown origin; (b) a sample from the specific source specified by the prosecution hypothesis; and (c) a collection of samples from the alternative source population. One approach is to assume that the alternative source population is completely known and rely on a Bayes Factor for deciding between the competing hypotheses. In this presentation, we will relax this assumption and explore some of the resulting issues arising from the estimation of the alternative source population. We will illustrate the resulting effects with a collection of glass fragment data.