Yglesias attempts to do the math on the probability of finding a terrorist:

The other point is that monitoring the UK’s 1.5 million Muslims is a lost cause. If you have a 99.9 percent accurate method of telling whether or not a given British Muslim is a dangerous terrorist, then apply it to all 1.5 million British Muslims, you’re going to find 1,500 dangerous terrorists in the UK. But nobody thinks there are anything like 1,500 dangerous terrorists in the UK. I’d be very surprised if there were as many as 15. And if there are 15, that means you’re 99.9 percent accurate method is going to get you a suspect pool that’s overwhelmingly composed of innocent people. The weakness of al-Qaeda’s movement, and the very tiny pool of operatives it can draw from, makes it essentially impossible to come up with viable methods for identifying those operatives.

Yglesias proves that he knows nothing about quantitative reasoning. You do not use a single screening test to find terrorists. As a number of filters are applied, the number of false positives goes down substantially. So there is no need for it to have 99.99% accuracy. Do we have such filters? I do not know. But to say that we need a "99.9 percent accurate method" is ignorant.

Moreover whatever happened to logical reasoning? Since when did we start treating 1.5 million people as suspects? Surely there must be some criteria which shows what socio - economic groups will show a greater tendency to become terrorists.