by Charles L. Hooper
Hoover Institution
November 14, 2013
Should governments spy on their own citizens to find and neutralize terrorists? For a diagnostic test to be considered a good screening tool for the general population, experts say it should have a positive predictive value (PPV) of 5 to 25 percent. There are 317 million Americans. Assume that the NSA’s methods have a sensitivity of 80 percent and a specificity of 80 percent. If we use the formula for PPV and target a PPV of one percent, then the NSA would only be justified in spying on us if more than 800,000 Americans—roughly the population of South Dakota—were terrorists. Clearly, the real number is far below 800,000. With these numbers, the NSA is absolutely not justified in spying on Americans.

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