A researcher follows 1,000 factory workers for 2 years to measure the incidence of noise-induced hearing loss. During follow-up, 40 workers develop hearing loss contributing 3,800 person-years, while the remaining workers contribute 16,200 person-years. What is the incidence density of hearing loss in this cohort?

• A 2 per 1,000 person-years
• B 20 per 1,000 person-years ✓
• C 25 per 1,000 person-years
• D 40 per 1,000 person-years

Explanation

Incidence density equals number of new cases divided by total person-time of observation. Total person-years are 3,800 plus 16,200, giving 20,000 person-years. Forty cases divided by 20,000 person-years equals 0.002 per person-year, or 20 per 1,000 person-years. Option D uses only the number of cases without dividing by person-time, which is not a rate.

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

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