… or why data mining is a good way to find stolen credit cards and not at all a good way to find terrorists:

No matter how sophisticated and super-duper are NSA’s methods for identifying terrorists, no matter how big and fast are NSA’s computers, NSA’s accuracy rate will never be 100% and their misidentification rate will never be 0%. That fact, plus the extremely low base-rate for terrorists, means it is logically impossible for mass surveillance to be an effective way to find terrorists.

– via Bruce Schneier, who knows a thing or two about this stuff.