Are Inconclusive Decisions in Forensic Science as Deficient as They Are Said to Be?

The inconclusive category, entropy, and forensic firearm identification

There has been extensive recent discussion of the difficulty in estimating meaningful error rates in forensic firearms examinations, and other areas of pattern evidence. The 2016 President's Council of Advisors on Science and Technology (PCAST) report was clear in criticizing many forensic disciplines as lacking the types of studies that would provide error rate measurements seen in other scientific fields. However, there is a substantial lack of consensus on the approach to measuring an "error rate" for fields such as forensic firearm examination that include in the conclusion scale the "inconclusive" category, as occurs in the Association of Firearm and Tool Mark Examiners (AFTE) Range of Conclusions and many other such fields. Many authors appear to assume the error rate calculated in the binary decision model is the only appropriate way to report errors, but there have been attempts made to adapt the error rate from the binary decision model to scientific fields in which the inconclusive category is viewed as a meaningful outcome of the examination process. In this study we present three neural networks of differing complexity and performance trained to classify the outlines of ejector marks on cartridge cases fired from different firearm models, as a model system for examining the performance of various metrics of error in systems using the inconclusive category. We also discuss an entropy, or information, based method to assess the similarity of classifications to ground truth that is applicable to range of conclusion scales, even when the inconclusive category is used.

Accuracy of comparison decisions by forensic firearms examiners

This black box study assessed the performance of forensic firearms examiners in the United States. It involved three different types of firearms and 173 volunteers who performed a total of 8640 comparisons of both bullets and cartridge cases. The overall false-positive error rate was estimated as 0.656% and 0.933% for bullets and cartridge cases, respectively, while the rate of false negatives was estimated as 2.87% and 1.87% for bullets and cartridge cases, respectively. The majority of errors were made by a limited number of examiners. Because chi-square tests of independence strongly suggest that error probabilities are not the same for each examiner, these are maximum-likelihood estimates based on the beta-binomial probability model and do not depend on an assumption of equal examiner-specific error rates. Corresponding 95% confidence intervals are (0.305%, 1.42%) and (0.548%, 1.57%) for false positives for bullets and cartridge cases, respectively, and (1.89%, 4.26%) and (1.16%, 2.99%) for false negatives for bullets and cartridge cases, respectively. The results of this study are consistent with prior studies, despite its comprehensive design and challenging specimens.

Planning, design and logistics of a decision analysis study: The FBI/Ames study involving forensic firearms examiners

This paper describes design and logistical aspects of a decision analysis study to assess the performance of qualified firearms examiners working in accredited laboratories in the United States in terms of accuracy (error rate), repeatability, and reproducibility of decisions involving comparisons of fired bullets and cartridge cases. The purpose of the study was to validate current practice of the forensic discipline of firearms/toolmarks (F/T) examination. It elicited error rate data by counting the number of false positive and false negative conclusions. Preceded by the experimental design, decisions, and logistics described herein, testing was ultimately administered 173 qualified, practicing F/T examiners in public and private crime laboratories. The first round of testing evaluated accuracy, while two subsequent rounds evaluated repeatability and reproducibility of examiner conclusions. This project expands on previous studies by involving many F/T examiners in challenging comparisons and by executing the study in the recommended double-blind format.

Forensic science and the principle of excluded middle: "Inconclusive" decisions and the structure of error rate studies

In a paper published recently in this journal, Dror and Scurich (2020) [20] critically discuss the notions of "inconclusive evidence" (i.e., test items for which it is difficult to render a categorical response) and "inconclusive decisions" (i.e., experts' conclusions or responses) in the context of forensic science error rate studies. They expose several ways in which the understanding and use of "inconclusives" in current forensic science research and practice can adversely affect the outcomes of error rate studies. A main cause of distortion, according to Dror and Scurich, is what they call "erroneous inconclusive" decisions, in particular the lack of acknowledgment of this type of erroneous conclusion in the computation of error rates. To overcome this complication, Dror and Scurich call for a more explicit monitoring of "inconclusives" using a modified error rate study design. Whilst we agree with several well-argued points raised by the authors, we disagree with their framing of "inconclusive decisions" as potential errors. In this paper, we argue that referring to an "inconclusive decision" as an error is a contradiction in terms, runs counter to an analysis based on decision logic and, hence, is questionable as a concept. We also reiterate that the very term "inconclusive decision" disregards the procedural architecture of the criminal justice system across modern jurisdictions, especially the fact that forensic experts have no decisional rights in the criminal process. These positions do not ignore the possibility that "inconclusives" - if used excessively - do raise problems in forensic expert reporting, in particular limited assertiveness (or, overcautiousness). However, these drawbacks derive from inherent limitations of experts rather than from the seemingly erroneous nature of "inconclusives" that needs to be fixed. More fundamentally, we argue that attempts to score "inconclusives" as errors amount to philosophical claims disguised as forensic methodology. Specifically, these attempts interfere with the metaphysical substrate underpinning empirical research. We point this out on the basis of the law of the excluded middle, i.e. the principle of "no third possibility being given" (tertium non datur).

Interpol review of fingermarks and other body impressions 2016-2019

This review paper covers the forensic-relevant literature in fingerprint and bodily impression sciences from 2016 to 2019 as a part of the 19th Interpol International Forensic Science Managers Symposium. The review papers are also available at the Interpol website at: https://www.interpol.int/content/download/14458/file/Interpol%20 Review%20 Papers%202019. pdf.