A cohort of 800 industrial workers is followed for 5 years to determine the incidence of contact dermatitis. One hundred workers leave employment after exactly 3 years and are then lost to follow-up. The remaining 700 complete the full 5 years. In total, 38 new cases of dermatitis occur during the study. What is the incidence density of dermatitis?

• A 9.5 per 1000 population
• B 47.5 per 1000 person-years
• C 10.0 per 1000 person-years ✓
• D 12.7 per 1000 person-years

Explanation

The 100 workers lost at 3 years contribute 100 x 3 = 300 person-years. The remaining 700 contribute 700 x 5 = 3500 person-years. Total person-time = 3800 person-years. Incidence density = 38 / 3800 x 1000 = 10 per 1000 person-years. Option B results from wrongly dividing by the number of cases times follow-up, and option D ignores the denominator contribution of dropouts entirely.

Reference: Gordis Epidemiology, 6th ed.

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