In DNA Numbers Game, Statistics Could Lead to False Convictions


For more than three decades, the rape and murder of Diana Sylvester, a San Francisco nurse, went unsolved. Then, in 2004, a search of California’s DNA database of criminal offenders yielded an apparent breakthrough: Badly deteriorated DNA from the assailant’s sperm was linked to John Puckett, an obese, wheelchair-bound 70-year-old with a history of rape. The DNA “match” was based on fewer than half of the genetic markers typically used to connect someone to a crime, and there was no other physical evidence. Puckett insisted he was innocent, saying that although DNA at the crime scene happened to match his, it belonged to someone else.

At Puckett’s trial earlier this year, the prosecutor told the jury that the chance of such a coincidence was 1 in 1.1 million. Jurors were not told, however, the statistic that leading scientists consider the most significant: the probability that the database search had hit upon an innocent person. In Puckett’s case, it was 1 in 3. The case is emblematic of a national problem, the Los Angeles Times has found. Prosecutors and crime labs across the country routinely use numbers that exaggerate the significance of DNA matches in “cold hit” cases, in which a suspect is identified through a database search. “It is only a matter of time until someone is wrongfully convicted because of this,” said Keith Devlin, a Stanford mathematician.


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