Over the summer, the Texas Forensic Science Commission, which sets standards for physical evidence in state courts, came to an unsettling conclusion: There was something wrong with how state labs were analyzing DNA evidence.
It seemed the labs were using an outdated protocol for calculating the probability of DNA matches in “mixtures”; that is, crime scene samples that contain genetic material from several people. It may have affected thousands of cases going back to 1999.
At first, they assumed the update wouldn’t make a big difference — just a refinement of the numbers.
But when a state lab reran the analysis of a DNA match from a murder case about to go to trial in Galveston, Texas, it discovered the numbers changed quite a bit.
Under the old protocol, says defense layer Roberto Torres, DNA from the crime scene was matched to his client with a certainty of more than a million to one. That is, you’d have to go through more than a million people to find somebody else who’d match the sample. But when the lab did the analysis again with the new protocol, things looked very different.
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