In a prospective occupational cohort, 5 workers developed silicosis after a total of 2,500 person-years of observation among dust-exposed employees. The incidence rate of silicosis in this cohort is:
- A 2 per 1,000 person-years ✓
- B 0.2 per 100 person-years
- C 20 per 1,000 person-years
- D 5 per 2,500 persons at risk
Correct answer: A. 2 per 1,000 person-years
Explanation
Incidence density equals number of new cases divided by total person-time of observation: 5 divided by 2,500 person-years gives 0.002 per person-year, expressed as 2 per 1,000 person-years. Option D restates the raw counts without converting to a rate and ignores the rate concept entirely. Person-time denominators are preferred when follow-up durations differ between individuals.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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