Suppose a test for a disease is 80% accurate for those who have the disease (true positives) and 80% accurate for those who do not have the disease (true negatives). Within a sample of 4000 patients, the incidence rate of the disease matches the national average, which is 1.5%.

a. Of those with the disease, what percentage test positive?
b. Of those who test positive, what percentage have the disease?

Compare this result to the one from part (a), and explain why they are different.

c. Suppose a patient tests positive for the disease. As a doctor using this table, how would you describe the patient’s chance of actually having the disease? Compare this figure to the overall incidence rate of the disease.

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Disease No disease Total Test positive 48 788 836 Test negative 12 3152 3164 Total 60 3940 4000