The dirty truth about DNA results and why the FBI doesn't want the information made public.

Typically, law enforcement and prosecutors rely on FBI estimates for the rarity of a given DNA profile—a figure can be as remote as one in many trillions when investigators have all thirteen markers to work with. In Puckett’s case, where there were only five and a half markers available, the San Francisco crime lab put the figure at one in 1.1 million—still remote enough to erase any reasonable doubt of his guilt. The problem is that, according to most scientists, this statistic is only relevant when DNA material is used to link a crime directly to a suspect identified through eyewitness testimony or other evidence. In cases where a suspect is found by searching through large databases, the chances of accidentally hitting on the wrong person are orders of magnitude higher.

The reasons for this aren’t difficult to grasp: consider what happens when you take a DNA profile that has a rarity of one in a million and run it through a database that contains a million people; chances are you’ll get a coincidental match. Given this fact, the two leading scientific bodies that have studied the issue—the National Research Council and the FBI’s DNA advisory board—have recommended that law enforcement and prosecutors calculate the probability of a coincidental match differently in cold-hit cases. In particular, they recommend multiplying the FBI’s rarity statistic by the number of profiles in the database, to arrive at a figure known as the Database Match Probability. When this formula is applied to Puckett’s case (where a profile with a rarity of one in 1.1 million was run through a database of 338,000 offenders) the chances of a coincidental match climb to one in three.
Such coincidental matches are more than a theoretical possibility, as Chicago police can attest. In 2004, detectives investigating a string of robberies on the city’s North Side found some skin cells that the culprit had left behind at one crime scene, which contained six DNA markers. When they ran this profile against Illinois’s offender database, they found it matched a woman named Diane Myers. There was just one problem: when the burglaries in question were committed, Myers was already in jail, serving time on drug charges.

Indeed, the little information that has come to light about the actual rate of coincidental matches in offender databases suggests the chances of hitting on the wrong person may be even higher than the Database Match Probability suggests. In 2005, Barlow heard that an Arizona state employee named Kathryn Troyer had run a series of tests on the state’s DNA database, which at the time included 65,000 profiles, and found multiple people with nine or more identical markers. If you believe the FBI’s rarity statistics, this was all but impossible—the chances of any two people in the general population sharing that many markers was supposed to be about one in 750 million, while the Database Match Probability for a nine-marker match in a system the size of Arizona’s is roughly one in 11,000.

Barlow decided to subpoena Troyer’s searches, believing the finding would be helpful for a case she was working on. To her surprise, she discovered that Troyer had unearthed not just a couple of pairs who shared nine identical markers, but 122. "That was a ‘wow’ moment," Barlow recalls.

As it turns out, these findings were no fluke. Searches of databases elsewhere have revealed similarly unsettling numbers. In 2006, for instance, a Chicago judge ordered a search of the Illinois database, which contained 233,000 profiles. It turned up 903 pairs with nine or more matching DNA markers. Among geneticists and statisticians, these findings have eroded faith in the FBI’s DNA rarity statistics, which were based on data from just 200 or 300 people and are used by crime labs across the country. Laurence Mueller, an ecology and evolutionary biology professor at University of California, Irvine, told me that anyone who knows statistics finds the figures "laughable."

Rather than try to sort out the disparities between its numbers and database findings, the FBI has fought to keep this information under wraps. After Barlow subpoenaed the Arizona database searches, the agency sent the state’s Department of Public Safety a cease-and-desist letter, warning that its conduct was "under review." Eventually, the Arizona attorney general obtained a court order to block Barlow’s distribution of the findings. In other instances, the FBI has threatened to revoke access to the bureau’s master DNA database if states make the contents of their systems available to defense teams or academics. Agency officials argue they have done so because granting access would violate the privacy of the offenders (although researchers generally request anonymous DNA profiles with no names attached) and tie up the FBI’s computers, impeding investigations. These justifications baffle researchers. In the December 2009 issue of the journal Science, dozens of biologists, geneticists, and forensic experts urged the FBI to change its secretive policy, saying that there was no way that allowing a handful of researchers to run database searches, each of which takes only a few minutes, would hamper investigations. They also dismissed the agency’s privacy concerns, saying, "The government frequently releases sensitive information under controlled conditions to verified researchers." Krane of Wright State University, who was the letter’s lead author, believes the real reason the FBI has blocked access is to avoid revealing the shortcomings in its own system. "Analysis of the offender database is sure to expose the misconceptions and errors in the method the FBI used to arrive at its rarity statistics," he told me.




Link: http://www.washingtonmonthly.com/features/2010/1003.bobelian.html