In a cohort study of smoking and myocardial infarction in City A,the number of myocardial infarctions were 250 cases among 7,000 smokers and the number of myocardial infarctions were 150 cases among 20,000 nonsmokers.From another source,a cross-sectional study,we know that 30% of the total population in City A were smokers.What is the population proportion attributable risk of smoking in City A?

A) 15%
B) 30%
C) 53%
D) 79%
E) 90%

The population proportion attributable risk (PPAR) equals the incidence in the total population minus the incidence in the nonexposed divided by the incidence in the total population.In this data,the total population incidence is 35.7 times 0.3 plus 7.5 times 0.7.This gives a total population incidence of 16.0 per 1,000 persons.Using the total population incidence in the PPAR calculation then gives an estimate of 53%.This is found by subtracting 7.5 from 16.0 and dividing this result by 16.0.