In a prospective cohort study, the incidence rate among exposed individuals is 45 per 1000 person-years and among unexposed is 15 per 1000 person-years. What is the attributable risk percent (ARP) among the exposed?

• A 33%
• B 200%
• C 75%
• D 67% ✓

Explanation

Attributable risk percent among the exposed = (Ie − Iu)/Ie × 100 = (45−15)/45 × 100 = 30/45 × 100 = 66.7% ≈ 67%. This means approximately 67% of the disease in the exposed group is attributable to the exposure. The relative risk here is 3, but ARP uses the formula (RR−1)/RR × 100 = 2/3 × 100 = 67%, confirming the answer. 33% would be the unexposed fraction; 75% would correspond to RR=4.

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

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