FBI resists scrutiny of 'matches'

Soon, defense lawyers in other states were seeking what came to be known as "Arizona searches."

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'Don't panic'

For years, DNA's strength in the courtroom has been the brute power of its numbers. It's hard to argue with odds like 1 in 100 billion.

Troyer's discovery threatened to turn the tables on prosecutors. At first blush, the Arizona matches appeared to contradict those statistics and the popular notion that DNA profiles, like DNA, were essentially unique.

Law enforcement experts scrambled to explain.

Three months after the court-ordered search in Arizona, Steven Myers, a senior DNA analyst at the California Department of Justice, gave a presentation to the Assn. of California Crime Lab Analysts. It was titled "Don't Panic" -- a hint at the alarm Troyer's discovery had set off.

Many of the Arizona matches were predictable, Myers said, given the type of search Troyer had conducted.

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. 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.

When this "database effect" was considered, about 100 of the 144 matches Troyer had found were to be expected statistically, Myers found.

Troyer's search also looked for matches at any of 13 genetic locations, while in a real criminal case the analyst would look for a particular profile -- making a match far less likely.

Further, any nonmatching markers would immediately rule out a suspect. In the case of the black and white men who matched at nine loci, the four loci that differed -- if available from crime scene evidence -- would have ensured that the wrong man was not implicated.

The presence of relatives in the database could also account for some of Troyer's findings, the FBI and other experts say. Whether that's the case would require cumbersome research because the databases don't contain identifying information, they say.

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Flaws in assumptions?

Some scientists are not satisfied by these explanations. They wonder whether Troyer's findings signal flaws in the complex assumptions that underlie the FBI's rarity estimates.