If a test to detect a disease whose prevalence is one in a thousand has a false positive rate of 5%, what is the chance that a person found to have a positive result actually has the disease, assuming you know nothing about the person's symptoms or signs?¹

--pages 3-4, 7 of Randomness
1. Cassells, Schoenberger, and Grayboys, 1978 (p. 999) In this case, we are assuming that the test correctly diagnoses the disease every time; that is, there are no false negatives.

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