In a cohort of industrial workers, 12 new cases of noise-induced hearing loss occur over a total observation period of 6000 person-years. The incidence density of hearing loss is:

• A 12 per 6000 workers
• B 2 per 1000 person-years ✓
• C 0.5 per 100 person-years
• D 20 per 1000 person-years

Explanation

Incidence density equals number of new cases divided by total person-time of observation: 12 divided by 6000 person-years gives 0.002 per person-year, which converts to 2 per 1000 person-years. Option A restates the raw counts without computing a rate, option C is off by a factor of four, and option D is the reciprocal arithmetic error of dividing person-years by cases. Person-time denominators properly account for workers lost to follow-up or entering late, unlike a simple cumulative incidence.

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

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