Are DNA Tests as Accurate as We Thought?
Trumpeted for years as virtually infallible, DNA tests may not be as accurate as we thought.
Questions raised by an Arizona lab worker who stumbled across several close DNA matches that seemingly defied the odds of a match, as described in court, have led to more extensive efforts to test DNA databases for accuracy in a number of states, reports the Los Angeles Times reported in detail this week in a lengthy article about the issue.
Those efforts are opposed by the Federal Bureau of Investigation laboratory that administers the national DNA testing system. It contends that the accuracy checks–which involve searches for matches between all DNA profiles, rather than matches to one specific profile–are misleading and unhelpful, and may violate the privacy of individuals whose profiles are being examined. It has also threatened to shut down labs that continue cooperating with such accuracy checks, the newspaper writes.
But analyst Kathryn Troyer points out that a number of labs have commonly matched suspects based on a nine-point (“location on chromosome” or “locus”) comparison of their DNA profiles. In fact, a number of suspects may match at this level, leading to the possibility of a mistaken identification, she says.
At the request of Bicka Barlow, a San Francisco defense attorney defending a suspect in a rape-murder from 20 years earlier, a California court subpoenaed the Arizona database, seeking to determine how common such matches are among the population of suspects for which it has DNA samples.
“Among about 65,000 felons, there were 122 pairs that matched at nine of 13 loci. Twenty pairs matched at 10 loci. One matched at 11 and one at 12, though both later proved to belong to relatives,” the Times reports.
The reason for the matches is clear when you understand the different way in which comparisons are made when searching for a suspect and searching for similar DNA profiles throughout the database:
“In a database search for a criminal case, a crime scene sample would have been compared to every profile in the database–about 65,000 comparisons,” the Times article explains. “But Troyer compared all 65,000 profiles in Arizona’s database to each other, resulting in about 2 billion comparisons. Each comparison made it more likely she would find a match.”
Hat tip: How Appealing
