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DNA: GENES AS EVIDENCE

When a match is far from a lock

The odds of an error are greater than you might think in 'cold hit' cases, which tap databases to ID suspects.

May 04, 2008|Jason Felch and Maura Dolan | Times Staff Writers

Police found the naked body of Diana Sylvester near her Christmas tree.

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.

For The Record
Los Angeles Times Thursday, May 29, 2008 Home Edition Main News Part A Page 2 National Desk 2 inches; 82 words Type of Material: Correction
DNA evidence: A May 4 article in Section A about the statistical calculations involved in describing DNA evidence in a murder case contained an arithmetic error. It said that multiplying the probability of 1 in 1.1 million by 338,000 was the same as dividing 1.1 million by 338,000. Actually, it's the same as dividing 338,000 by 1.1 million. The answer, a 1 in 3 probability of a coincidental match between crime scene DNA and genetic profiles in a state database, was correct.
For The Record
Los Angeles Times Sunday, June 01, 2008 Home Edition Main News Part A Page 2 National Desk 2 inches; 80 words Type of Material: Correction
DNA evidence: A May 4 article in Section A about the statistical calculations involved in describing DNA evidence in a murder case contained an arithmetic error. It said that multiplying the probability of 1 in 1.1 million by 338,000 was the same as dividing 1.1 million by 338,000. Actually, it's the same as dividing 338,000 by 1.1 million. The answer, a 1-in-3 probability of a coincidental match between crime scene DNA and genetic profiles in a state database, was correct.

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 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 dozens of interviews with leading scientific and legal authorities, a comprehensive review of scientific and academic papers in the field and court documents in cold hit cases.

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.

The FBI lab, which oversees the nation's offender databases, has disregarded the recommendation of its own advisory board, bureau officials acknowledged.

So far, the courts have ruled in law enforcement's favor on this issue.

As a result, some experts fear, a technology best known for freeing the innocent could be causing its own miscarriages of justice.

"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.

DNA profiles are widely perceived as a unique genetic fingerprint. In fact, they are slivers of the human genome -- up to 13 markers that contain about a millionth of the information on all the chromosomes. Relatives often share many markers, and even unrelated people on average share two or three.

So DNA "matches" by themselves can never definitively link someone to a crime.

The best science can do is to estimate the likelihood that a match has occurred by sheer chance. These statistics are easily distorted or misunderstood by lawyers, judges, juries and even expert witnesses.

This potential for distortion is compounded in cold hit cases.

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.

But 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 individual 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.

The problem will only grow as the nation's criminal DNA databases expand. They already contain 6 million profiles.

The danger of implicating an innocent person through a cold hit becomes particularly acute with crimes that are decades old, like Sylvester's killing. Witnesses have died, evidence has been lost and memories have faded. All of this puts greater weight on the DNA evidence, which also may have deteriorated.

Prosecutors and crime-lab analysts say they don't mention the database problem to juries because it could unfairly reveal that a defendant has a criminal record.

They also say that using more conservative statistics would not make a difference in many cases, because the odds would still overwhelmingly favor the prosecution. Some don't seem to understand the problem or rely on experts who disagree with the scientific mainstream.

Now the California Supreme Court is weighing the statistical and other concerns raised by cold hit cases -- the highest court yet to consider them. People vs. Nelson, which involves the 1976 murder of a college student in Sacramento, is scheduled for oral arguments Thursday.

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