Significance of DNA ‘matches’ in cases not always definitive

By Jason Felch and Maura Dolan | The 22-year-old San Francisco nurse had been sexually assaulted and stabbed in the heart. She lay on her back, her neck laced with scratches and her mouth open as if frozen in a scream.

For more than three decades, Sylvester’s slaying 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.

Jurors are often told that the odds of a coincidental match are hundreds of thousands of times more remote than they actually are, according to a review of scientific literature and interviews with leading authorities in the field.

Two national scientific committees, including the FBI’s DNA advisory board, have recommended portraying the odds more conservatively. But interviews with expert witnesses and DNA analysts from crime labs across the country show that few if any have adopted that approach.

“It is only a matter of time until someone is wrongfully convicted because of this,” said Keith Devlin, a Stanford mathematician who has studied the problem.

For years, police used DNA only to compare a crime-scene sample to a single person they had other reasons to suspect. In court, prosecutors could legitimately cite the remote odds that someone selected at random off the street would share the same genetic profile.

In cold hit cases, the investigation starts with a DNA match found by searching thousands, or even millions, of genetic profiles in an offender database. Each comparison increases the chance of a match to an innocent person.

Nevertheless, police labs and prosecutors almost always calculate the odds as if the suspect had been selected randomly from the general population in a single try.

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