Validity and reliability of forensic firearm examiners

Abstract

Forensic firearm examiners compare the features in cartridge cases to provide a judgment addressing the question about their source: do they originate from one and the same or from two different firearms? In this article, the validity and reliability of these judgments is studied and compared to the outcomes of a computer-based method. The features we looked at were the striation patterns of the firing pin aperture shear marks of four hundred test shots from two hundred Glock pistols, which were compared by a computer-based method. Sixty of the resulting 79,800 comparisons were shown to 77 firearm examiners. They were asked to judge whether the cartridge cases had the same source or a different source, and to indicate the degree of support the evidence provided for those judgments.The results show that the true positive rates (sensitivity) and the true negative rates (specificity) of firearm examiners are quite high. The examiners seem to be slightly less proficient at identifying same-source comparisons correctly, while they outperform the used computer-based method at identifying different-source comparisons.The judged degrees of support by examiners who report likelihood ratios are not well-calibrated. The examiners are overconfident, giving judgments of evidential strength that are too high. The judgments of the examiners and the outcomes of the computer-based method are only moderately correlated.We suggest to implement performance feedback to reduce overconfidence, to improve the calibration of degree of support judgments, and to study the possibility of combining the judgments of examiners and the outcomes of computer-based methods to increase the overall validity.