In a historical study, the death rate among heavy smokers was 224 per 1 lakh person-years while among non-smokers was 10 per 1 lakh person-years. The death rate in the general population was 74 per 1 lakh person-years. Which one among the following best represents the attributable risk of smoking for mortality in the given scenario?
95.5%
The correct answer is 95.5%. Attributable risk percent among the exposed = [(Incidence in exposed − Incidence in non-exposed) ÷ Incidence in exposed] × 100. Substituting: incidence in heavy smokers = 224 and in non-smokers = 10 per 100000 person-years. Excess risk = 224 − 10 = 214 per 100000 person-years. AR% = (214 ÷ 224) × 100 = 95.5% — that is, about 95.5% of deaths in heavy smokers are due to smoking. (Relative risk here = 224/10 = 22.4.)
Why the others are wrong: 22.4% misreads the relative risk of 22.4 as a percentage. 86.5% comes from wrongly using the general-population rate as denominator: (74 − 10)/74 = 86.5%, which is the population attributable risk percent, not AR in the exposed. 67.0% corresponds to no valid calculation; the figure of 74 is a deliberate distractor.
Key point: AR% uses the exposed group as denominator; PAR% uses the total population rate. The denominator alone separates these two answers.
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