Firearm examination: Examiner judgments and computer-based comparisons
Erwin J. Mattijssen, Cilia Witteman, Charles Berger, Xiaoyu A. Zheng, Johannes A. Soons, Reinoud Stoel
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 comparisons sets. We also compared the computer-based methods comparison scores with the examiners degree of support judgments and assessed the validity of the examiners degree of support judgments. The results of this study show that the source decisions of the computer-based method and the examiners were quite valid (TPR = 94.4% and TNR > 91.7% for the computer-based method and TPR > 95.3% and TNR > 86.2% for the examiners). Validity of 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. Correlation between the computer-based methods comparison scores and the examiners degree of support judgments was low for the same-source comparisons and 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 degrees 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.