Five workers are followed for up to 8 years in an occupational cohort. One develops silicosis at 3 years, one is lost to follow-up at 4 years without disease, and the remaining three remain disease-free through the full 8 years. The incidence density of silicosis in this group is approximately:

• A 12.5 per 100 person-years
• B 32.3 per 1000 person-years ✓
• C 20 per 100 person-years
• D 3.2 per 100 person-years

Explanation

Person-time equals 3 plus 4 plus three times 8, giving 31 person-years. Incidence density is 1 case divided by 31 person-years, which is 0.0323 per person-year, or about 32.3 per 1000 person-years. Option A incorrectly divides 1 by 8 years, option D misplaces the decimal by two orders, and option C treats loss to follow-up as contributing no meaningful denominator adjustment.

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

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