How the NSA scours the internet in one handy graphic
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Terrorists are rare. To estimate the number of actual killer terrorists in the country (most of whom haven't actually done anything yet) let's estimate 9/11 as a major effort that used perhaps 10% of them. That would give us 200 committed terrorists in the population.
Let's say that the NSA created an incredible technique that is accurate at identifying a terrorist in collected (sorry "touched"
info 99.9% of the time. That is probably much better than what can be accomplished to be honest.
The 99.9% success rate means they would find almost all 200 terrorists easily: (0.999 * 200) = 199.8.
But the algorithm also fails 0.1% of the time. In the US population of 316 million that means that (0.001 * 316 million) = 316000 Americans falsely under suspicion as well.
In the end, a given person fingered by a 99.9% accurate algorithm as a terrorist would only be guilty 0.06% of the time.
This seems to have been understood after 9/11....
http://www.nytimes.com/2006/01/17/politics/17spy.html?oref=login
