More TSA Stupidity

From Effect Measure, some interesting observations on the futility of behavioral detection protocols at airports:

Suppose that behavioral detection were so sensitive that you picked every single terrorist trying to get on an airplane. There are about 750 million passenger trips this year, so let's say one in ten million involves a terrorist, or 75 terrorist trips. Let's also say that the behavioral test IDs them all (highly unlikely) but also makes a mistake in about one in a ten thousandth of a percent of passenger trips. That's 750 false positives. That means for behavioral detector whose skills are 100% sensitive and 99.9999% specific (meaning he tags the wrong person only once per million times) the positive predictive value would still be only 10%. So it isn't at all surprising that the PPV of the current system is functionally zero and probably exactly zero. It's a fool's errand.