In a cohort of 100 workers followed for variable durations, 5 workers were each followed for 2 years before developing disease, 15 workers were each followed for 4 years without disease, and the remaining 80 workers were each followed for 1 year without disease. What is the incidence density (person-time incidence rate) of disease?

• A 5 per 170 person-years
• B 5 per 160 person-years ✓
• C 50 per 1000 person-years
• D 5 per 100 person-years

Explanation

Person-time is calculated by summing observation time until disease onset or censoring. The 5 cases contribute 5 x 2 = 10 person-years. The 15 disease-free workers contribute 15 x 4 = 60 person-years, and the 80 workers contribute 80 x 1 = 80 person-years, giving a total of 150 plus 10 equals 160 person-years. The rate is therefore 5 cases per 160 person-years. Option C incorrectly converts without computing true denominator time.

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

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