A cohort of 1000 factory workers is followed for 5 years. During follow-up, 50 workers develop occupational asthma and several drop out early, so the total observation time accumulated is 4500 person-years. The incidence density of asthma is:

• A 5.0 per 1000 person-years
• B 10.0 per 1000 person-years
• C 11.1 per 1000 person-years ✓
• D 50.0 per 1000 person-years

Explanation

Incidence density = number of new cases divided by total person-time of observation = 50 / 4500 person-years = 0.0111 per person-year, that is 11.1 per 1000 person-years. This method correctly handles unequal follow-up caused by losses to follow-up and staggered entry, unlike cumulative incidence which uses only the initial denominator. Option B would apply if all 1000 were observed for the full 5 years (5000 person-years).

Reference: Gordis Epidemiology, 6th ed.

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