No, his calculation shows P(baddy|flag)=1/10,000. In other words, given that the system flags somebody, there is only a 1/10,000 chance that this person was co

No, his calculation shows P(baddy|flag)=1/10,000. In other words, given that the system flags somebody, there is only a 1/10,000 chance that this person was correctly flagged. This is given the absurdly high P(flag|baddy) of .99 (a terrorist raises a flag 99% of the time... good luck with that.) and a P(flag|innocent person) of .01 (meaning that in the US an absolutely staggering 3.19 million people would be wrongfully flagged by this system). Then there is P(baddy)=1/1,000,000, which implies that we believe there are 300+ terrorists currently in the US that desperately need to be caught. What the hell have those 300+ terrorists been doing in the past few years? Wishing they could only find guns the the great US of A? 1/10,000,000 is a far more reasonable number, though even 1/100,000,000 is probably being generous. 1/10,000 is absolutely abysmal. When you start punching in more realistic numbers though the result becomes much worse and you rapidly approach a situation where you would have more luck flagging random US citizens as uncaught criminals than you will with using this system to identify terrorists. If you permit those sort of absurd odds, then even hiring "forensic psychics" to investigate murders in small towns starts to make sense... This sort of screening flat out does not work for something as rare as terrorists. It only works when you start bumping your P(baddy) way up (dropping P(flag|innocent person) is not practical, and raising P(flag|baddy) to even more absurdly high levels only gets you marginal improvements). Replace 'terrorist' with 'person of interest to the DEA' or 'tax cheat' and suddenly this sort of screening starts to look a hell of a lot more practical.... hmm....