Wire-cutting forensics are currently too unreliable in court, says new research

A research article published on June 10 in the Proceedings of the National Academy of Sciences emphasizes the importance of careful application of high-tech forensic science to prevent wrongful convictions.

In a study with implications for a range of forensic investigations that rely on “massive databases and efficient algorithms,” researchers found that the chance of a false match increases significantly when investigators make millions of comparisons in a quest to match wires found at a crime scene with the tools that would have been used to cut them.

The rate of incorrect identifications could be as high as one in 10 or more, concluded the researchers, who are with the Center for Statistics and Applications in Forensic Evidence (CSAFE), based in Ames, Iowa.

“It’s somewhat counterintuitive,” says co-author Susan VanderPlas, an assistant professor of statistics at the University of Nebraska-Lincoln. “You’re more likely to find the right match, but you’re also more likely to find the wrong match.”

VanderPlas worked as a research professor at CSAFE before moving to Nebraska in 2020. Co-authors of the study, “Hidden Multiple Comparisons Increase Forensic Error Rates,” were Heike Hoffmann and Alicia Carriquiry, both with CSAFE and the Department of Statistics at Iowa State University. .

Wire breaks and tool marks are often used as evidence in robberies, bombings and other crimes. In the case of wire cutting, small stripes on the cut ends of a wire can be matched with one of the many tools available in a toolbox or garage. Comparing the evidence with more instruments increases the chance that similar streaks will be found on unrelated instruments, resulting in a false charge and conviction.

Wire-cutting evidence has been in dispute in at least two cases that have attracted national attention, including one in which the suspect was linked to a bomb attack based on a small piece of wire, only a fraction of a diameter in diameter. centimeter, which was linked to a bomb attack. tools found among the suspect’s belongings.

“Wire-cutting evidence is used in court and based on our findings it should not be – at least not without providing additional information about the number of comparisons made,” VanderPlas said.

Threading evidence is evaluated by comparing the striations on the cut end of a piece of wire to the cutting blades of tools suspected of being used in the crime. In a manual test, the experimenter slides the end of the wire along the path created along another piece of material cut with the same tool to see where the pattern of stripes matches.

An automated process uses a comparison microscope and pattern matching algorithms to find possible matches pixel by pixel.

This can result in thousands upon thousands of individual comparisons depending on the length of the cutting blade, the diameter of the wire and even the number of tools checked.

For example, VanderPlas said she and her husband added up the various tin snips, wire cutters, pliers and similar tools stored in their garage to arrive at a blade length totaling 23 feet.

Examiners may not even be aware of the number of comparisons they make while searching for a matching pattern, because those comparisons are hidden in the algorithms.

“This often ignored issue increases the number of false discoveries and may contribute to the erosion of public confidence in the justice system through the conviction of innocent individuals,” the study authors wrote.

Forensic investigators typically testify based on subjective rules about how much similarity is needed to make an identification, the study explained. The researchers were unable to obtain error rate studies for wire cross-section studies and used published error rates for ballistic studies to estimate possible false discovery rates for wire cross-section studies.

Before wire-cutting studies are used as evidence in court, researchers recommended the following:

• Examiners report the total length or surface area of ​​the materials used in the examination process, including blade length and wire diameter. This would allow error rates to be calculated for the entire study.
• Studies are being conducted to assess both false discovery and false elimination error rates when researchers make difficult comparisons. Studies should link the length and area of ​​the comparison to error rates.
• The number of items searched, comparisons made, and results returned must be reported when a database is used at any stage of the forensic evidence evaluation process.

The VanderPlas article echoes other reports calling for improvements in forensic science in America. The National Academies Press, publisher of the PNAS journal and other publications of the National Academies of Sciences, Engineering and Medicine, also published the groundbreaking 2009 report “Strengthening Forensic Science in the United States: A Path Forward.”