John Allen Paulos explains why TIA is an incredibly bad idea even if it works perfectly.

To see why this is so and to make the calculations easy, let's postulate a population of 300 million people of whom 1,000 are future terrorists. The system will correctly identify, we're assuming, 99 percent of these 1,000 people as future terrorists. Thus, since 99 percent of 1,000 is 990, the system will apprehend 990 future terrorists. Great! They'll be locked up somewhere.

But wait. There are, by assumption, 299,999,000 non-terrorists in our population and the system will be right about 99 percent of them as well. Another way of saying this is that it will be wrong about 1 percent of these people. Since 1 percent of 299,999,000 equals 2,999,990, the system will swoop down on these 2,999,990 innocent people as well as on the 990 guilty ones, incarcerating them all.