In a cohort study following 1000 smokers and 1000 non-smokers over 20 years, 200 smokers and 50 non-smokers develop COPD. The attributable risk percent among exposed is closest to:
- A 75% ✓
- B 80%
- C 150%
- D 25%
Correct answer: A. 75%
Explanation
Incidence in exposed = 200/1000 = 0.20; incidence in unexposed = 50/1000 = 0.05. Attributable risk = 0.20 - 0.05 = 0.15. Attributable risk percent (Levin's attributable risk) among exposed = (AR/Incidence in exposed) × 100 = (0.15/0.20) × 100 = 75%. This means 75% of COPD in smokers is attributable to smoking. Option C (150%) is the relative risk, not the AR%; option B (80%) and D (25%) are incorrect calculations.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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