A cohort of 400 factory workers is followed for 16 months to detect new onset of contact dermatitis. During follow-up, 10 workers develop dermatitis. The total observation time accrued by the cohort is 5,600 worker-months. The incidence density of dermatitis in this cohort, expressed per 1,000 worker-months of observation, is:
- A 0.18
- B 1.8 ✓
- C 178
- D 18
Correct answer: B. 1.8
Explanation
Incidence density equals number of new cases divided by total person-time of observation: 10 divided by 5,600 worker-months, multiplied by 1,000, giving 1.8 per 1,000 worker-months. Option C comes from multiplying instead of dividing, option D omits multiplication by 1,000 correctly scaled, and option A results from an arithmetic slip. Park's Textbook of Preventive and Social Medicine describes this rate as the preferred measure when follow-up times vary between subjects.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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