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Study Raises Alarms About Inaccuracies and Bias in Gun Forensics Reporting

by Jo Ellen Nott

An October 2023 study from Iowa State University reveals a troubling trend among firearms experts reporting on cartridge-case comparisons. The authors of the study, Gary Wells and Andrew Smith, state that gun forensic examiners are improperly labeling mismatches as “inconclusive,” potentially concealing evidence that could convict or exonerate innocent defendants.

The researchers examined a dataset of nearly 2,000 comparisons and found that 32 percent of actual mismatches were reported as inconclusive, compared to only one percent of matches. This lopsidedness suggests something beyond human error and indicates potential bias on the part of firearms forensic examiners, according to the study.

The authors of the study believe a flawed response scale is to blame for the potential bias. The response scale asks examiners if casings are from the “same source,” a question open to misinterpretation when firearm alterations or degraded evidence are present. Some examiners, out of an abundance of caution, may automatically select the inconclusive option rather than falsely risk excluding a gun from a crime.

This cautiousness on the part of firearms forensics examiners can harm the defense and leave innocent individuals at risk of prolonged investigations or even wrongful convictions. Wells and Smith recommend a more straightforward approach—asking examiners if the shell casings from the suspect’s gun match those found at the crime scene and then providing separate questions about potential alterations to provide greater transparency.

A further complication is the potential for adversarial allegiance bias. With most examiners funded by prosecution or police, there is a risk of inconclusive reports favoring their “employer” even when evidence points towards innocence. This concern is amplified by the fact that some labs have policies forbidding mismatch reports.

Wells and Smith recommend several actions be taken by the courts to safeguard the fair administration of justice. First and very importantly, defense attorneys must challenge inconclusive reports and demand that examiners “show their work.” Additionally, when inconclusive findings are provided, defense attorneys must seek a second opinion. Already decided cases with inconclusive reports must be reexamined to ensure innocent defendants were not unfairly implicated.

The two researchers also give recommendations to firearms forensic science: The field must prioritize good practices that minimize bias. To more accurately report their findings of gun casing comparisons, experts must create a new response scale. The field also needs to address conflicts of interest regarding who funds them.

The study points out that forensic science should serve both sides of the courtroom by delivering evidence that not only convicts the guilty but also exonerates the innocent. Taking steps to mitigate the flaws Smith and Wells found in their study will help bring about a more impartial and effective delivery of justice.   


Source: Smith, Andrew; Wells, Gary. “Telling Us Less Than What They Know: Expert Inconclusive Reports Conceal Exculpatory Evidence in Forensic Cartridge-Case Comparisons. Journal of Applied Research in Memory and Cognition (2023)

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