Monthly Archives: June 2013

Thomas Bayes looks through the NSA’s “Prism”

Leaked NSA slide-deck claims that NSA has “direct access” to servers at Google, Apple, Facebook, Skype, Yahoo, and many others

This story has received a lot of coverage in the last few days, even extending to Jay Leno’s monologue. Understandably, there is a lot of concern about the civil liberties implications. But there is also another question, answered by Corey Chivers: How likely is the NSA PRISM program to catch a terrorist?

We don’t really know anything about how PRISM works (NSA = Never Say Anything), but with some plausible assumptions we can estimate the answer. Suppose

  1. If a terrorist is in the system, the probability is 99% that PRISM will flag him/her.
  2. An good guy has only a 1/100 chance of being flagged as potential terrorist.
  3. The actual number of terrorists is quite small, say 1 in 1,000,000.

Using Bayes’ rule, Chivers shows that only 1 in 10,102 of the people flagged as suspects will actually be a terrorist!
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