Base Rate Neglect: Why Your "95% Accurate" Alert Is Wrong 99% of the Time

In 1978, researchers asked 60 people at Harvard Medical School a single question. A disease affects 1 in 1,000 people. The test for it has a 5% false positive rate. A patient tests positive. What's the chance they actually have it? The most common answer was 95%. The right answer is about 2%.