How Rare Is One in 113 Billion?

At the NY Times Freakonomics blog, economist Steven Levitt goes through the math behind the FBI's DNA match probabilities. Recently, an Arizona state crime lab analyst found two unrelated men who matched at nine of 13 loci -- the FBI estimates the odds of such a match to be one in 113 billion. With a hypothetical chance of two people matching at any one locus at 7.5 percent, the odds of two people matching at nine loci is one in 13 billion. "The bottom line is that DNA testing is not perfect, but it is still a million (or maybe a thousand?) times better than anything else we have to catch criminals," writes Levitt.