In a prospective cohort of 800 factory workers followed for 4 years to study noise-induced hearing loss, 32 workers develop hearing loss. No workers are lost to follow-up. What is the incidence density of hearing loss in this cohort?

• A 1.0 per 100 person-years ✓
• B 4.0 per 100 person-years
• C 8.0 per 100 person-years
• D 0.01 per 100 person-years

Explanation

Incidence density equals new cases divided by total person-time. Total person-time is 800 workers multiplied by 4 years, giving 3200 person-years. Dividing 32 cases by 3200 person-years gives 0.01 cases per person-year, expressed as 1.0 per 100 person-years. Option B comes from dividing cases by number of workers alone (cumulative incidence of 4%) rather than by person-time, which is the classic error this item tests.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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