Matt Yglesias wrote a piece mathematically apologizing for failed security efforts. In short, he claims that given Britain's population size, a 99.9% method of detecting terrorists would yield 1500 suspects, whilst he thinks only 15 actually exist. Given the huge number of false positives, identifying the actual 15 becomes very difficult.

He overlooks the monitoring capabilities of the Brits (or maybe just has a very realist appreciation of their abilities ;). With a 99.9% accurate model, one would rejoice and happily put the 1500 people under surveillance (aperiodically trail them for a day; monitor communications, contacts; always search them at airports; etc.). I'd probably be a cheeky bastard and provided a model of a good citizen, I'd tell a test subset of the suspects that they are on the airport search list and that to get off the search list they should do any combination of the good citizen attributes (married with kids, start a company, etc.) and see whether that changes their communications and contacts.

There is a reason why the foreign businessman who travels plenty and has a local wife (and preferable some sproglets) is more likely to be on the watchlist than the bum of a teenager who makes loud inquiries about the availability of C4. :) I would like an idea of what sort of computing power is needed to monitor the flood of data ECHELON brings in though. --CL