Validity of forensic cartridge-case comparisons

Max Guyll · Stephanie Madon · Yueran Yang · Kayla A. Burd · Gary L. Wells

24 citations0/0 files storedProceedings of the National Academy of Sciences

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Legal processes and jurisprudence Deception detection and forensic psychology Gun Ownership and Violence Research

Abstract

This article presents key findings from a research project that evaluated the validity and probative value of cartridge-case comparisons under field-based conditions. Decisions provided by 228 trained firearm examiners across the US showed that forensic cartridge-case comparison is characterized by low error rates. However, inconclusive decisions constituted over one-fifth of all decisions rendered, complicating evaluation of the technique's ability to yield unambiguously correct decisions. Specifically, restricting evaluation to only the conclusive decisions of identification and elimination yielded true-positive and true-negative rates exceeding 99%, but incorporating inconclusives caused these values to drop to 93.4% and 63.5%, respectively. The asymmetric effect on the two rates occurred because inconclusive decisions were rendered six times more frequently for different-source than same-source comparisons. Considering probative value, which is a decision's usefulness for determining a comparison's ground-truth state, conclusive decisions predicted their corresponding ground-truth states with near perfection. Likelihood ratios (LRs) further showed that conclusive decisions greatly increase the odds of a comparison's ground-truth state matching the ground-truth state asserted by the decision. Inconclusive decisions also possessed probative value, predicting different-source status and having a LR indicating that they increase the odds of different-source status. The study also manipulated comparison difficulty by using two firearm models that produce dissimilar cartridge-case markings. The model chosen for being more difficult received more inconclusive decisions for same-source comparisons, resulting in a lower true-positive rate compared to the less difficult model. Relatedly, inconclusive decisions for the less difficult model exhibited more probative value, being more strongly predictive of different-source status.

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References

2011Accuracy and reliability of forensic latent fingerprint decisions 2008The ‘elasticity’ of criminal evidence: a moderator of investigator bias 2020(Mis)use of scientific measurements in forensic science 2018“Cannot Decide”: The Fine Line Between Appropriate Inconclusive Determinations Versus Unjustifiably Deciding Not To Decide 2008Fingerprint Error Rates and Proficiency Tests: What They are and Why They Matter 2014A Study of False-Positive and False-Negative Error Rates in Cartridge Case Comparisons 2022Accuracy of comparison decisions by forensic firearms examiners 2020The Error in ‘Error Rate’: Why Error Rates Are So Needed, Yet So Elusive

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2024Inconclusive decisions and error rates in forensic science 2024Methodological problems in every black-box study of forensic firearm comparisons 2023Scientific guidelines for evaluating the validity of forensic feature-comparison methods

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