A screening test has sensitivity 90 percent and specificity 95 percent. It is applied to 10 000 people in a population where 1 percent have the disease. Approximately what is the positive predictive value?
Answer and explanation
Correct answer: C. 15 percent
- Step 1: Count true and false positives. Diseased: 1 percent of 10 000 = 100.
- Step 2: True positives = 90 percent of 100 = 90. Healthy = 9900, and 5 percent of these = 495 false positives. PPV = 90 divided by 585 = about 15 percent.
Answer
15 percent.
Why not the others: 50 percent: A binary outcome does not mean equal probability. 90 percent: Sensitivity is not predictive value, which depends on prevalence. 95 percent: Specificity is not predictive value.
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