Calculating positive predictive value
A test is given to 1,000 people, 100 of whom have the disease. It is positive in 90 of the 100 with disease and negative in 810 of the 900 without it. What is its positive predictive value?
Answer B: explanation for every choice
A
10% is the disease prevalence.
B
Correct. True positives = 90, and false positives = 900 − 810 = 90. PPV = 90/(90 + 90) = 50%.
C
90% is both the sensitivity (90/100) and the specificity (810/900).
D
About 99% is the negative predictive value: 810/(810 + 10).
TakeawayPPV = TP/(TP + FP). It falls when prevalence is low.
Equation usedSensitivity, specificity, and predictive values
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