Firearm examination: Examiner judgments and computer‐based comparisons

Journal of Forensic Sciences, vol. 66, pp. 96–111

Abstract

Forensic firearm examination provides the court of law with information about the source of fired cartridge cases. We assessed the validity of source decisions of a computer-based method and of 73 firearm examiners who compared breechface and firing pin impressions of 48 comparison sets. We also compared the computer-based method's comparison scores with the examiners' degree-of-support judgments and assessed the validity of the latter. The true-positive rate (sensitivity) and true-negative rate (specificity) of the computer-based method (for the comparison of both the breechface and firing pin impressions) were 94.4% and at least 91.7%, respectively. For the examiners, the true-positive rate was at least 95.3% and the true-negative rate was at least 86.2%. The validity of the source decisions improved when the evaluations of breechface and firing pin impressions were combined and for the examiners also when the perceived difficulty of the comparison decreased. The examiners were reluctant to provide source decisions for "difficult" comparisons even though their source decisions were mostly correct. The correlation between the computer-based method's comparison scores and the examiners' degree-of-support judgments was low for the same-source comparisons to negligible for the different-source comparisons. Combining the outcomes of computer-based methods with the judgments of examiners could increase the validity of firearm examinations. The examiners' numerical degree-of-support judgments for their source decisions were not well-calibrated and showed clear signs of overconfidence. We suggest studying the merits of performance feedback to calibrate these judgments.

Authors 6

1. Erwin J.A.T. Mattijssen corresponding

   Radboud University Nijmegen · Netherlands Forensic Institute

   Affiliation as printed

   Behavioural Science Institute Radboud University Nijmegen Nijmegen The Netherlands

   Netherlands Forensic Institute The Hague The Netherlands

   Behavioural Science Institute, Radboud University Nijmegen, Nijmegen, The Netherlands

   Netherlands Forensic Institute, The Hague, The Netherlands

2. Cilia L. M. Witteman

   Radboud University Nijmegen

   Affiliation as printed

   Behavioural Science Institute Radboud University Nijmegen Nijmegen The Netherlands

   Behavioural Science Institute, Radboud University Nijmegen, Nijmegen, The Netherlands

3. Charles E.H. Berger Aachen Institute for Criminal Law and Criminology Leiden University Leiden The Netherlands Institute for Criminal Law and Criminology

   Leiden University · Netherlands Forensic Institute

   Affiliation as printed

   Institute for Criminal Law and Criminology Leiden University Leiden The Netherlands

   Netherlands Forensic Institute The Hague The Netherlands

   Institute for Criminal Law and Criminology, Leiden University, Leiden, The Netherlands

   Netherlands Forensic Institute, The Hague, The Netherlands

4. Xiaoyu A. Zheng

   National Institute of Standards and Technology

   Affiliation as printed

   Sensor Science Division National Institute of Standards and Technology Gaithersburg Maryland USA

   Sensor Science Division, National Institute of Standards and Technology, Gaithersburg, Maryland, USA

5. Johannes A. Soons

   National Institute of Standards and Technology

   Affiliation as printed

   Sensor Science Division National Institute of Standards and Technology Gaithersburg Maryland USA

   Sensor Science Division, National Institute of Standards and Technology, Gaithersburg, Maryland, USA

6. Reinoud D. Stoel

   Netherlands Forensic Institute

   Affiliation as printed

   Netherlands Forensic Institute The Hague The Netherlands

   Netherlands Forensic Institute, The Hague, The Netherlands