In a prospective cohort of 4 workers followed for contact dermatitis, Worker A was observed for 5 years without developing disease, Worker B developed the disease after 3 years of follow-up, Worker C was lost to follow-up after 2 disease-free years, and Worker D developed the disease after 4 years. The incidence density of dermatitis in this cohort is:

• A 20 per 100 person-years
• B 28.6 per 100 person-years
• C 14.3 per 100 person-years ✓
• D 50 per 100 person-years

Explanation

Incidence density uses person-time as the denominator. Each worker contributes follow-up time only until the event of interest: A contributes 5 years, C contributes 3 years (follow-up stops at diagnosis), B contributes 2 years (stops at loss), and D contributes 4 years, giving 14 person-years total. With 2 incident cases, the rate is 2 divided by 14, multiplied by 100, which equals 14.3 per 100 person-years. Option B results from incorrectly counting C and D beyond their diagnosis dates.

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

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Written and medically reviewed by the StethoPrep medical team.