Brain Teasers & Mental Math
1% of a population has a disease. A test is 90% accurate both ways (90% sensitivity, 90% specificity). You test positive. What is the probability you actually have the disease?
Model answer
1/12 ≈ 8.3% — far lower than intuition suggests.
Use a population of 1,000: 10 people are sick → 9 test positive (true positives). 990 are healthy → 10% test positive anyway = 99 false positives. Of…
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