A screening test is used in same way in two similar populations, but the proportion of false positive results among those who test positive in population A is lower than among those who test positive in population B. What is the likely explanation for this finding?
Correct Answer: The prevalence of disease is higher in population A
Description: Total population having the disease : ‘a + c’ (TP + FN)
Total population not having the disease : ‘b + d’ (FP + TN)
Total population: a + b + c + d = TP + FP + FN + TN
PPV depends on sensitivity, specificity and prevalence of disease in the population.
Now in this question, a screening test is used in same way in two similar populations; thereby the screening test will have similar sensitivity and specificity in both populations.
PPV = a/ (a + (b) X 100, thus b (False Positive rate) is inversely proportional to PPV; and PPV is directly proportional to Prevalence of disease in a population.
So, False Positive rate (FP rate) is inversely proportional to the prevalence of disease in the population.
Therefore, if the same screening test is having lower FP rate in population A (as compared to a similar population B), then this could be explained by higher prevalence of disease in population A
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Social & Preventive Medicine
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