In a city of twenty million like New York, there might be one or two terrorists. Maybe ten of them at the outside. 10/20,000,000 = 0.00005 percent. One twenty-thousandth of a percent. That's pretty rare all right. Now, say you've got some software that can sift through all the bank records, or toll pass records, or public transit records, or phone call records in the city and catch terrorists 99 percent of the time. In a pool of twenty million people, a 99 percent accurate test will identify two hundred thousand people as being terrorists. But only ten of them are terrorists. To catch ten bad guys, you have to haul in and investigate two hundred thousand innocent people.