Thursday, August 10, 2006

Precision, recall, and bottles of water.

The latest airline explosion plot has sparked a predictable overreaction by the much-beloved TSA, which once again demonstrates that it lacks a basic understanding of precision and recall.

Let's recap, shall we ? Suppose you're trying to find all elements of a subset S in a universe U, and return as an answer the set H.

Precision measures the quality of your answer; what fraction of the elements of H are indeed in S ? The higher the precision, the lower the false-positive rate.

Recall measures the efficacy of the procedure; what fraction of the elements of S did you actually find ? The higher the recall, the lower the false-negative rate.

As any statistician will tell you, false-positives and false-negatives are complementary; increase one, and the other decreases. Search engines need high precision, (they also need good ranking, but that's a different story).

The TSA is clearly going for high recall. Banning bottles of water will surely eliminate any future plans for liquid explosives that use water, but it also eliminates the many (how shall I say) *innocent* uses of water ?

p.s I don't mind the short term enforcement of stricer guidelines while law enforcement attempts to figure out what kinds of explosives were being designed. I just have no faith that the new draconian regulations will be relaxed any time soon, considering how long it took to allow us to carry nail clippers on board flights again.


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