In a prospective cohort of 500 factory workers followed for 2 full years, 25 workers develop occupationally related dermatitis. Assuming no losses to follow-up, what is the incidence rate of dermatitis?

• A 2.5 cases per 1,000 person-years
• B 12.5 cases per 1,000 person-years
• C 25 cases per 1,000 person-years ✓
• D 50 cases per 1,000 person-years

Explanation

Total person-time equals 500 workers multiplied by 2 years, giving 1,000 person-years. The incidence rate is 25 cases divided by 1,000 person-years, expressed as 25 per 1,000 person-years. Option A results from wrongly dividing by 10,000 person-years, option B from dividing by 2,000 person-years, and option D from using 500 person-years. Indian exam convention expresses such rates per 1,000 person-years unless stated otherwise.

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

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