What a Positive Test Really Means

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Test 1,000 people in a 3D town square for a made-up infectious disease, gather only the positives on a stage and reveal their actual status to see the conditional probability P(disease | positive) as counts of people, a 2×2 table and a formula. Change prevalence, sensitivity and specificity, test again, and use the multiplication rule to see why even an accurate test can give more false positives than true positives.

Key conceptsconditional probability · multiplication rule · sample space · complementary event · sensitivity · specificity · false positive · disease testing

• Probability & Statistics