In a cohort study of 500 factory employees, 400 remained under follow-up for the full 4 years and 100 were lost to follow-up at the end of year 2. Eighteen workers developed occupational asthma over the period. The incidence density (person-time incidence rate) of asthma is:

• A 9.0 per 1000 person-years
• B 10.0 per 1000 person-years ✓
• C 12.0 per 1000 person-years
• D 36.0 per 1000 person-years

Explanation

Total person-years equal 400 workers times 4 years plus 100 workers times 2 years, giving 1800 person-years. Incidence density is 18 divided by 1800, which equals 10 per 1000 person-years. Option A comes from wrongly counting 2000 person-years, and option C comes from ignoring the losses entirely and dividing by 1500.

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

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