How to make better forensic decisions

Abstract

Much of forensic practice today involves human decisions about the origins of patterned sensory evidence, such as tool marks and fingerprints discovered at a crime scene. These decisions are made by trained observers who compare the evidential pattern to an exemplar pattern produced by the suspected source of the evidence. The decision consists of a determination as to whether the two patterns are similar enough to have come from the same source. Although forensic pattern comparison disciplines have for decades played a valued role in criminal investigation and prosecution, the extremely high personal and societal costs of failure—the conviction of innocent people—has elicited calls for caution and for the development of better practices. These calls have been heard by the scientific community involved in the study of human information processing, which has begun to offer much-needed perspectives on sensory measurement, discrimination, and classification in a forensic context. Here I draw from a well-established theoretical and empirical approach in sensory science to illustrate the vulnerabilities of contemporary pattern comparison disciplines and to suggest specific strategies for improvement.

Commentary on: Monson KL, Smith ED, Peters EM. Accuracy of comparison decisions by forensic firearms examiners. J Forensic Sci. 2022;68(1):86–100. https://doi.org/10.1111/1556‐4029.15152.

“…However justified Monson et al may be in concluding that their results "add to the ever increasing body of empirical data that firearms examiners conduct comparisons with a high level of accuracy" [1]-and it may be that their conclusion cannot stand [4,6,20]-the data on poor performers in their study call into doubt whether opinions reached by firearms examiners can or should ever be admitted as evidence in any given criminal proceeding. The rules of evidence in both federal and state courts across the United States require judges, not only to assess the admissibility of particular methods, but also to evaluate, on a case-by-case basis, whether a given witness is "qualified" to testify about an accepted method because their knowledge or skills exceed those of everyday people, thereby allowing them to "assist the trier of fact" by accurately formulating opinions that would be beyond the capacity of lay jurors [14][15][16][17][18][19].…”

Reconsideration of Bertillonage in the age of digitalisation: Digital anthropometric patterns as a promising method for establishing identity

“…Forensic identity matching involves the comparison of evidence recovered from a crime scene with evidence obtained from a suspected source to determine whether they are similar enough to stem from the same source [ 4 ]. The emphasis is put on similarity since no two exemplars from the same source will ever be exactly identical due to differences in the circumstances of their acquisition [ 1 , 5 ].…”

The most consistent finding in forensic science is inconsistency

“…careful scrutiny because their heavy reliance on human judgment means they are especially vulnerable to human error, inconsistency across examiners, and cognitive bias" (emphasis added) [29]. See PCAST recommendation #2 [29], as well as [30,31].…”

“…“The more objective the approach, the more agreement there will be in the conclusions among examiners” [28] and “subjective methods require particularly careful scrutiny because their heavy reliance on human judgment means they are especially vulnerable to human error, inconsistency across examiners , and cognitive bias” (emphasis added) [29]. See PCAST recommendation #2 [29], as well as [30,31].…”