A Study of False-Positive and False-Negative Error Rates in Cartridge Case Comparisons

Abstract

Abstract : This report provides the details for a study designed to measure examiner (not laboratory) error rates for false identifications and false eliminations when comparing an unknown to a collection of three known cartridge cases. Volunteer active examiners with Association of Firearm and Toolmark Examiners (AFTE) membership or working in laboratories that participate in ASCLD were provided with 15 sets of 3 known + 1 unknown cartridge cases fired from a collection of 25 new Ruger SR9 handguns. The ammunition was all Remington 9-mm Luger (manufacturer designation L9MM3) and sets were made up of cartridge cases fired within 100 cartridges of each other for each gun. During the design phase of the experiment, examiners had expressed a concern that known samples should not be separated by a large number of fired cartridges. However, studies published on this effect indicate that several thousands of cartridges could be fired by the same firearm without making the identifying characteristics change enough to prevent identification. Examiners were provided with a background survey, an answer sheet allowing for the AFTE range of conclusions, and return shipping materials. They were also asked to assess how many of the 3 knowns were suitable for comparison, providing a measured rate of how often each firearm used in the study produces useable, quality marks. The participating examiners were provided with known positives and known negatives from independent groups of samples, providing independent measurements of a false-positive rate and independent measurements of a false-negative rate, allowing the study to measure both rates and uncertainties in those rates.