A Statistical Method for Matching a Bullet to a Cartridge Case

• Activity Number: 419
• Type: Contributed
• Date/Time: Tuesday, July 31, 2012 : 2:00 PM to 3:50 PM
• Sponsor: Section on Physical and Engineering Sciences
• Author(s): Tracy Morris*+ and Deion Christophe
• Companies: University of Central Oklahoma and University of Central Oklahoma
• Keywords: forensic science ; beta-binomial distribution ; fixed-bin analysis ; likelihood

Abstract

Firearm and toolmark examiners have historically relied on subjective criteria, related to an examiner's experience and training, to determine whether markings found on evidence from a crime scene can be matched to a source. Unfortunately, this experience and training are not enough to stand up in court. Consequently, it has become necessary to develop objective methods, supported by statistical evidence, by which examiners can match markings to tools. This research focuses on the question "Can a bullet be matched to the cartridge case from which it was fired?" Topographical bands of known matching and non-matching bullet pairs were examined for matching striations. The resulting data was used to estimate likelihoods of observing at least as many matching striations at random. This research has the potential to change the way in which examiners determine whether a bullet matches a cartridge case and to provide more defensible forensic evidence.