In an occupational cohort, 200 workers are followed for 1 year. Fifty workers leave employment at the midpoint of the year. Ten cases of dermatitis occur during follow-up. What is the incidence density of dermatitis in this cohort?
- A 50 per 1000 person-years
- B 66.7 per 1000 person-years
- C 57.1 per 1000 person-years ✓
- D 10 per cent per year
Correct answer: C. 57.1 per 1000 person-years
Explanation
Person-time contributed equals 150 workers followed a full year (150 person-years) plus 50 workers followed half a year (25 person-years), totalling 175 person-years. Incidence density = 10 divided by 175 = 5.71 per 100 person-years, that is 57.1 per 1000 person-years. Option B wrongly uses only the dropouts' nominal count, and option D ignores differential follow-up entirely, which is precisely what person-time corrects.
Reference: Gordis Epidemiology, 6th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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