# 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!

# Bayes’ Rule in daily life

Bayes’ Rule is a simple formula that tells you how to weigh evidence and change your beliefs. I don’t go around plugging numbers into a formula all the time, but nevertheless, becoming familiar with Bayes has shifted the way I think in some important ways.

# Monday Night Irish Class, June 3, 2013

Irish Class, JUne 3, 2013

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