Mismatched jeans –
Low reliability, high rate of false alarms in denim-matching forensics
The problems with forensic evidence — including fingerprint, bloodstain, and ballistics analysis — have terrible real-world consequences. According to the National Registry of Exonerations, nearly a quarter of wrongful convictions in the United States for the last (years can be attributed to flawed or misleading forensic evidence.
Computer scientists Sophie Nightingale and Hany Farid wanted to look at one technique in particular: photographic pattern analysis, which matches up the patterns of details on faces, hands, or clothing between suspects and crime-scene photographs. Jeans, for example, have a “barcode” pattern of dark and light splodges along their seams.
These patterns have been used as central evidence to convict people, but is this kind of analysis reliable? That hasn’t been established. To test it out, Nightingale and Farid went out to buy 111 pairs of jeans from second-hand stores. They laid the jeans out flat on a hard surface, photographed the seams along the legs, and digitally traced the pattern of light and dark points along the seams. To bump up their sample, they had Amazon Turk workers supply images from another pairs, photographed using careful instructions.
The important question, of course, is whether these patterns can be used to determine whether two images show the same pair of jeans . So the researchers took pairs of jeans and took photos of each, using different cameras, in different lighting, and with different draping. What they found was that any given pair of photos could come back with a lot of similarities but could also come back with very different readings on the pattern. The range was broad — as Nightingale and Farid point out, soft fabric photographed in a bunch of different ways is going to have distortions that vary from one image to the next.
False alarms
So, if one pair of jeans can look noticeably different in different photos, is denim-pattern analysis actually a useful forensic technique ? The researchers used their measurements to estimate how often a true match would come up and how often their jeans would throw up a “false alarm” —a score that looked like a match even though the images actually came from two different pairs.
This means that the technique of matching up jeans is likely to be pretty hit and miss — not catching actual similarities a lot of the time, and possibly throwing up a high rate of false alarms. And that’s under controlled experimental conditions using high-quality images and jeans laid out nice and flat, not grainy security footage showing jeans being worn. On the other hand, different features like damage, branding, and size could corroborate an analysis to improve the evidence one way or another.
GIPHY App Key not set. Please check settings