In a prospective cohort study, 500 non-smokers and 500 smokers are followed for 10 years. At the end, 20 non-smokers and 80 smokers develop COPD. What is the Population Attributable Risk Percent (PAR%) if the prevalence of smoking in the population is 30%?
- A Approximately 75%
- B Approximately 47% ✓
- C Approximately 60%
- D Approximately 30%
Correct answer: B. Approximately 47%
Explanation
Incidence in exposed (Ie) = 80/500 = 0.16; Incidence in unexposed (Iu) = 20/500 = 0.04. Incidence in total population (It) = Iu + Pe × (Ie − Iu) = 0.04 + 0.30 × (0.16 − 0.04) = 0.04 + 0.036 = 0.076. PAR% = (It − Iu) / It × 100 = (0.076 − 0.04) / 0.076 × 100 = 0.036/0.076 × 100 ≈ 47.4%. PAR% estimates the fraction of disease in the population attributable to the exposure, relevant to public health prioritisation.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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