Barlow had earned a master's degree in genetics from Cornell University before going to law school. A veteran of dozens of DNA cases, she considered Puckett's the weakest match she had ever seen.
Typically, prosecutors rely on FBI statistics to estimate the rarity of a particular DNA profile in the general population. This calculation is known as the Random Match Probability.
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.
The chance that two unrelated people will share the same 13 markers can be as remote as 1 in a quadrillion -- a number with 15 zeros. Because the match in Puckett's case involved only 5 1/2 genetic locations, the chance it was coincidental was higher but still remote: 1 in 1.1 million.
But Barlow thought this figure vastly exaggerated the strength of the evidence. It did not take into account how Puckett had been identified: through a search of a large database.
The general-population figures used by prosecutors portray the odds of matching crime-scene DNA to a single, randomly selected person.
But because database searches involve hundreds of thousands or millions of comparisons, experts say using the general-population statistic can be misleading.
Think of a lottery. If you buy a single ticket, your chances of hitting the jackpot are remote. If you buy many tickets, your odds improve with each purchase. In Barlow's view, the prosecution in effect bought hundreds of thousands of lottery tickets to find the match with Puckett. She contended that this greatly increased the odds of a match to an innocent person.
Barlow argued during pretrial hearings that the jury should be told about the recommendation of two leading panels of scientific experts, one convened by the National Research Council and the other by the FBI. Both committees settled upon a statistical remedy to adjust for the many individual comparisons made during a database search. It has been widely but not universally embraced by scientists.
In every cold hit case, the panels advised, police and prosecutors should multiply the Random Match Probability (1 in 1.1 million in Puckett's case) by the number of profiles in the database (338,000). That's the same as dividing 1.1 million by 338,000.
For Puckett, the result was dramatic: a 1-in-3 chance that the search would link an innocent person to the crime.
Barlow knew the concern was not just hypothetical. A database search had implicated an innocent man in Britain in 2001. A DNA profile of six markers from a crime scene had matched Raymond Easton, who was arrested and charged with robbery.
