When suspects are found by combing through large [DNA] databases, the odds are exponentially higher ... in some cases 1 in 3

... when police use fresh DNA material to link a crime directly to a suspect identified through eyewitness accounts or other evidence, the chances of accidentally hitting on an innocent person are extraordinarily slim. But when suspects are found by combing through large databases, the odds are exponentially higher. In Puckett’s case the actual chance of a false match is a staggering one in three, according to the formula endorsed by the FBI’s DNA advisory board and the National Research Council, a body created by Congress to advise the government and the public on scientific issues. But the jury that decided Puckett’s fate never heard that figure. In fact, his lawyers were explicitly barred from bringing it up.

... more than a million new profiles are being added to our nation’s offender databases each year, and as DNA testing becomes more routine, it is likely that these systems will grow to include an even wider cross-section of the public. Of course, as the number of profiles in the databases swell, so do the odds of accidentally fingering innocent people.