Forensic Analysis: Statistical Comparison of Bullet Lead Compositions

• Activity Number: 250
• Type: Other
• Date/Time: Tuesday, August 10, 2004 : 2:00 PM to 3:50 PM
• Sponsor: ASA
• Author(s): Karen Kafadar*+ and Cliff Spiegelman
• Companies: University of Colorado, Denver and Texas A&M University/The Texas Transportation Institute

Abstract

Since the 1960s, FBI has performed Compositional Analysis of Bullet Lead (CABL), a forensic technique that compares the elemental composition of bullets found at a crime scene to that of bullets found in a suspect's possession. CABL has been used when no gun is recovered, or when bullets are too small or fragmented to compare striations on the casings with those on the gun barrel. The National Academy of Sciences formed a Committee charged with the assessment of CABL's scientific validity. The report, "Forensic Analysis: Weighing Bullet Lead Evidence,'' included discussion on the effects of the manufacturing process on the validity of the comparisons, the precision and accuracy of the chemical measurement technique, and the statistical methodology used to compare two bullets and test for a "match." This talk will focus on the statistical analysis: the FBI's methods of testing for a "match,', the apparent false positive and false negative rates, the FBI's clustering algorithm ("chaining"), and the Committee's recommendations. Finally, additional analyses on data made available for future studies will be discussed.