It is estimated that the prevalence of smoking in a population is 50%. A cohort study conducted using a random sample from this population showed that the 5-year risk of ischemic stroke is 1:1000 among smokers and 0.5:1000 among non-smokers. What percentage of ischemic strokes observed in the population can be attributed to smoking?
A)10%
B)25%
C)33%
D)50%
E)75%

Question

It is estimated that the prevalence of smoking in a population is 50%.  A cohort study conducted using a random sample from this population showed that the 5-year risk of ischemic stroke is 1:1000 among smokers and 0.5:1000 among non-smokers. What percentage of ischemic strokes observed in the population can be attributed to smoking?
A)10%
B)25%
C)33%
D)50%
E)75%

Quizplus · Accepted Answer

Explanation: The percentage of ischemic strokes in the observed population that is attributable to smoking is given by the population attributable risk percent (PARP). Unlike attributable risk percent, PARP is the measure of excess risk in thetotal population, not only in exposed subjects. PARP compares 2 risks (or incidences): PARP = (Risk in total population - Risk in unexposed) / Risk in total population Given the risk of stroke among the exposed (0.1%) and the unexposed (0.05%), as well as the proportion of exposure in the population (0.5), the risk in the total population can be calculated as a weighted average: Risk in total population = (Risk in smokers)(Proportion of smokers) + (Risk in nonsmokers)(Proportion of nonsmokers) = (0.1%)(0.5) + (0.05%)(0.5) = 0.075%. Therefore, PARP = (0.075 - 0.05)/0.075 = 0.025/0.075 = 0.33 (33%). This means that 33% of ischemic strokes observed in the population can be attributed to smoking. (Alternately, the risk in the total population can be calculated by using a sample population. For example, in a sample population of 20,000 individuals where smoking prevalence is 50%, there are 10,000 smokers and 10,000 nonsmokers. The 5-year-risk of ischemic stroke is 0.1% among smokers and 0.05% among nonsmokers, so there will be (0.1%)(10,000) = 10 cases of ischemic stroke among the 10,000 smokers and (0.05%)(10,000) = 5 cases among the 10,000 nonsmokers. In sum, there will be 10 + 5 = 15 cases of ischemic stroke among the total population of 20,000, with a 5-year-risk of 15/20,000 = 0.075%. The result will be the same regardless of the sample population size.) A second way to calculate PARP uses the relative risk (RR): PARP = (Prevalence)(RR - 1) / [(Prevalence)(RR - 1) + 1] Prevalence = 0.5 and RR = 2.0 (as calculated previously), so PARP = 0.5 / (0.5 + 1) = 0.5/1.5 = 0.33 (ie, 33%). Educational objective: The population attributable risk percent (PARP) estimates the proportion of disease in the population that is attributed to the exposure. PARP = (Risk in the total population - Risk in the unexposed) / Risk in the total population.

It Is Estimated That the Prevalence of Smoking in a Population