In a prospective cohort study, the incidence rate of cardiovascular disease in smokers is 12 per 1000 person-years and in non-smokers is 4 per 1000 person-years. What is the Attributable Risk Percent (AR%) in the exposed group?

• A 33.3%
• B 75.0%
• C 80.0%
• D 66.7% ✓

Explanation

Attributable Risk (AR) = incidence in exposed − incidence in unexposed = 12 − 4 = 8 per 1000 person-years. AR% in exposed (Aetiologic fraction in exposed) = (AR / incidence in exposed) × 100 = (8/12) × 100 = 66.7%. This represents the proportion of disease in smokers attributable to smoking; the remaining 33.3% would occur regardless of smoking. This is distinct from the Population Attributable Risk Percent, which depends additionally on the prevalence of exposure in the population.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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