A cohort of 500 textile workers is followed for the development of byssinosis. During the study, 20 workers develop the disease. Some workers leave employment early, so the total follow-up accumulated is 4,500 person-years. What is the incidence density of byssinosis?

• A 4.44 per 1,000 person-years ✓
• B 4.0 per 1,000 population
• C 44.4 per 1,000 person-years
• D 0.44 per 1,000 person-years

Explanation

Incidence density equals number of new cases divided by total person-time of observation: 20 divided by 4,500 person-years, multiplied by 1,000, gives 4.44 per 1,000 person-years. Option B uses a denominator of population rather than person-time, which ignores variable follow-up duration. Person-time rates are preferred when participants are observed for unequal lengths or are lost to follow-up.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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