False positives in forensic comparison analysis are higher than they appear

What is it about?

Researchers report that increasing the number of comparisons in forensic analysis increases the potential for false positive matches, a finding demonstrated through the forensic analysis of instruments that produced cut wires. Forensic science relies on conclusively demonstrating that a suspected tool or person was responsible for the evidence in the crime scene. Susan Vanderplas and colleagues estimated the minimal number of comparisons between cut wires and cutting surfaces needed to conduct forensic analysis. The comparisons attempt to match the striations on the cut wire with the markings on the cutting surface. The authors note that the minimal number of comparisons is calculated as the width of the cutting surface divided by the wire diameter. However, the use of high-resolution digital imagery to examine cutting surfaces—a common practice in forensic science—can result in up to 20,000 comparisons. Using published estimates of false positive results, the authors found that even a modest number of comparisons can produce a false positive match. Increasing the number of comparisons increases the probability of a false positive match. According to the authors, the study carries implications for the likelihood of false positive matches in other types of forensic analysis, such as searches of ballistics databases for matching cartridge casing markings to firearms.

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Photo by Trust "Tru" Katsande on Unsplash

Photo by Trust "Tru" Katsande on Unsplash

Why is it important?

Higher error rates increase the likelihood that forensic evidence will be used to support a false conviction, that is, that an innocent person might be convicted and jailed.