A Retail Sampling Approach to Assess Impact of Geographic Concentrations on Probative Value of Comparative Bullet Lead Analysis

Abstract

The probative value of comparative bullet lead analysis (CBLA), a now discontinued technique that was used by the Federal Bureau of Investigation for more than 30 years, had been hotly debated for several years. One issue that had received relatively little attention concerned the degree of geographic dispersion of bullets as they pass from manufacturers to retailers. Proponents and critics of CBLA alike agree that geographic distribution is such a major consideration, if not a predominant one, that it could significantly diminish, or completely erode, the probative value of a CBLA 'match' or, in some cases, even make a match counter-probative. The inattention to this issue was a consequence of lack of data, rather than lack of importance. Until now, no data concerning bullet distribution had been presented in the public domain, critically hampering the proper estimation of the probative value of a CBLA match. In this paper, we use manufacturer packing codes on boxes of bullets in retail outlets at four sites in the United States as a surrogate measure of bullet lead compositions to gauge local retail bullet distribution. Using a weighted average packing code match probability, we found very high degrees of geographic concentration of bullet packing codes. Although these findings can only offer a rough estimate of the degree of geographic concentration of actual chemical compositions of bullets, they are sufficient to establish that geographic concentration does, in fact, exist. Such a concentration would have a significant impact on the probative value of any claimed CBLA match.

Keywords: comparative bullet lead analysis, compositional analysis of bullet lead, geographic concentration, geographic distribution, probative value, National Research Council, forensic bullet composition comparisons, forensic bullet evidence