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Base rate fallacy dooms TSA’s risk-based screening - Open Knowledge

May. 29th, 2012

12:52 am - Base rate fallacy dooms TSA’s risk-based screening

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Originally published at craschworks. You can comment here or there.

Base rate fallacy dooms TSA’s risk-based screening

Risk-based screening, if it describes any partitioning of airline passengers at checkpoints, is a fraud. It doesn’t matter what the criteria are for putting passengers into low-risk and high-risk categories, because the concept is mathematically unsound.

The base rate fallacy is the failure to account for underlying incidence of the condition being tested for. Imagine this hypothetical (and completely fantastical) scenario: that the TSA employs a near-perfect test for risk-based screening, a test that always flags terrorists if they are present and only flags innocent people 1 time out of 10,000. (The TSA’s actual success rate at flagging known terrorists is zero.)

Under these conditions, only one out of every five million people flagged would be a terrorist. This is the base rate fallacy in operation. In other words, for all intents and purposes, every flagged person is a false positive.”