A study conducted by researchers at Iowa State University has uncovered a bias in the reporting of firearm cartridge-case comparisons. The study reveals that forensic experts are more likely to report mismatches as “inconclusive” in cartridge-case comparisons, potentially leading to injustices in criminal cases. The researchers, Gary Wells and Andrew Smith, analyzed data from a previous experiment involving 228 firearms examiners and 1,811 cartridge-case comparisons.
The overall accuracy of the participants in determining whether casings matched or mismatched was found to be high. However, when applying a mathematical model to the data, Wells and Smith discovered that 32% of actual mismatch trials were reported as inconclusive, compared to only 1% of actual match trials.
The researchers suggest that a flawed response scale used by forensic firearms experts may contribute to this discrepancy. The current scale asks examiners whether the crime-scene casings and casings from the suspect’s gun are from the same source. Smith and Wells argue that this question fails to account for possible explanations such as altered firearms or degraded evidence, leading some examiners to default to calling results inconclusive.
Smith and Wells propose a revised response scale that simply asks examiners if the shell casings from the suspect’s gun match those found at the crime scene. They also recommend separate questions to address alterations and degradation. Until the response scale is fixed, the researchers advise defense lawyers to cross-examine forensic firearms experts who report inconclusive results and to seek a second opinion if the comparison report is inconclusive.
The study also highlights the potential influence of adversarial allegiance bias, where forensic firearm examiners and labs retained by the prosecution or police departments may be hesitant to report mismatches. Some labs even have policies that prohibit examiners from reporting mismatches. The researchers emphasize the need to minimize bias and improve transparency in cartridge-case comparisons to ensure a fair and efficient criminal justice system.