Strong opinions, weakly held

Fun with numbers

The Foreign Policy blog describes a new “terrorist detector” that the TSA is considering adding at airports. Basically, you sit in the machine and answer questions while hooked up to sensors of some kind. It uses answers and measurements to determine whether you’re “afraid of being caught.”

What’s interesting to me is that they claim an 85% success rate. That sounds atrocious to me. You can read that a number of ways, but let’s say they’re speaking plainly and the machine returns the correct result (“terrorist” or “not terrorist”). For starters, that means that the machine says that 1 in 6 or 1 in 7 people who sit in the machine will be flagged as a terrorist when they’re not.

There haven’t been any terrorist attacks on US domestic flights since 9/11 and there haven’t been any terrorist attacks on US bound flights since Richard Reid’s shoe bombing attempt. Is a machine that says that 1 in 6 travelers are probably terrorists really useful?

This, of course, is one of the many problems with racial profiling as well. Profile any race, age group, religion, or astrological sign and you still find that very few members of the group are terrorists, because the number of actual terrorists is miniscule.


  1. If it has a known false positive and false negative rate, then it is very useful — although perhaps not all by itself. If you had, say, four things like this with known error rates you could easily combine them using a little Bayesian math into something that was extremely accurate.

    BTW, the error rate you cite is for false negative. 15% of those pretending to be terrorists slipped through undetected. The false positive rate was 8%, so 8% of non-terrorists were incorrectly recommended for further questioning.

  2. “Afraid of being caught”, that’s the proposed parameters for policing passengers? Not only it impractical (if they implement this, I can swim across the Atlantic/Pacific and an aircraft’s worth of human cargo would still be shackled to sensors) but it’s sweeping into stupidity. Soon, they’ll be deeming adulteresses as terrorists, naughty kids as terrorists, common criminals as terrorists and on and on. Heck, we’ll all be flagged as extremists because we are “afraid of being caught” … by the damn detector. “…speed up and cheapen security procedures” Right, so by eluding technology (15% is expansive gap), I will be able to bring onboard explosive technology. Sweet!

  3. useful? for making us safe, no. for making the electorate scared, yes.

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