"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.
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.
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.
