Sep 7th, 2009

Excerpted from Little Brother by Cory Doctorow:

This is the paradox of the false positive, and here’s how it applies to terrorism:

Terrorists are really rare. 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.

Guess what? Terrorism tests aren’t anywhere close to 99 percent accurate. More like 60 percent accurate. Even 40 percent accurate, sometimes.

Read the entire book for free at Doctorow’s website.

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