A cohort study examining lung cancer risk among asbestos workers finds a Relative Risk of 3.8 (95% CI 2.1–6.9). When researchers calculate the Attributable Risk Percent (ARP) among the exposed, which formula correctly represents this measure?
- A (RR − 1) / RR × 100
- B (Ie − Iu) / Ie × 100
- C Both A and B are mathematically equivalent ✓
- D Ie − Iu expressed as absolute risk difference per 1000
Correct answer: C. Both A and B are mathematically equivalent
Explanation
Attributable Risk Percent among exposed (ARP) = (Ie − Iu)/Ie × 100, where Ie = incidence in exposed and Iu = incidence in unexposed. This is mathematically identical to (RR − 1)/RR × 100 because Ie/Iu = RR, so (Ie − Iu)/Ie = (RR − 1)/RR. Option D describes absolute attributable risk, not the percentage. Both A and B represent the same measure using different input data.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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