title: Objective Evaluation of Subclass Characteristics on Breech Face Marks.
abstract: Subclass characteristics can be found on the breech face marks left on spent cartridge cases. Even if they are assumed to be rare and their reported number is small, they can potentially lead to false associations. Subclass characteristics have been studied empirically allowing examiners to recognize them and to understand in which conditions they are produced. Until now, however, their influence on the identification process has not been studied from a probabilistic point of view. In this study, we aim at measuring the effect of these features on the strength of association derived from examinations involving subclass characteristics. The study takes advantage of a 3D automatic comparison system allowing the calculation of likelihood ratios (LRs). The similarities between cartridge case specimens fired by thirteen S&W .40S&W Sigma pistols are quantified, and their respective LRs are computed. The results show that the influence of subclass characteristics on the LRs is limited, even when these features are prevalent among the potential sources considered in a case. We show that the proportion of firearms sharing subclass characteristics should be larger than 40% of the pool of potential firearms for the effect to be significant.
authors.affiliation: School of Criminal Sciences, University of Lausanne, 1025, Lausanne-Dorigny, Switzerland.|Institute of Legal Medicine, University of Bern, Bühlstrasse 20, Bern, 3012, Switzerland.
authors.forename: Fabiano
authors.initials: F
authors.lastname: Riva
authors.affiliation: Netherlands Forensic Institute, PO Box 24044, 2490 AA, The Hague, The Netherlands.
authors.forename: Rob
authors.initials: R
authors.lastname: Hermsen
authors.affiliation: Netherlands Forensic Institute, PO Box 24044, 2490 AA, The Hague, The Netherlands.
authors.forename: Erwin
authors.initials: E
authors.lastname: Mattijssen
authors.affiliation: Netherlands Forensic Institute, PO Box 24044, 2490 AA, The Hague, The Netherlands.
authors.forename: Pascal
authors.initials: P
authors.lastname: Pieper
authors.affiliation: School of Criminal Sciences, University of Lausanne, 1025, Lausanne-Dorigny, Switzerland.
authors.forename: Christophe
authors.initials: C
authors.lastname: Champod
keywords: 3D topographies; cartridge cases; evaluation; firearm identification; forensic science; likelihood ratio; subclass characteristics