Telling Us Less Than What They Know: Expert Inconclusive Reports Conceal Exculpatory Evidence in Forensic Cartridge-Case Comparisons

Abstract

Cartridge-case comparison experts are tasked with determining whether a reference sample associated with the suspect originated from the same source as the evidence sample found at the crime scene. Same-source reports have strong inculpatory value and different-source reports have strong exculpatory value. Surprisingly, inconclusive reports have strong exculpatory value as they occur much more frequently for actual nonmatches than for actual matches. We used a signal detection model to test our hypothesis that some experts report inconclusive when they detect a nonmatch. Consistent with this hypothesis, we found that even when examiners were able to perfectly discriminate matches from nonmatches, they rendered inconclusive reports on 32% of nonmatch trials. Experts are biased to avoid rendering different-source reports, which conceals exculpatory information that innocent persons desperately need to establish their innocence. We argue that this biased examiner might result from an adversarial allegiance bias combined with a flawed response scale.