A prospective cohort study follows 5000 non-hypertensive individuals aged 40–60 for 10 years. At the end of the study, 500 develop hypertension. The person-time at risk is 42,000 person-years (accounting for dropouts). The Incidence Density Rate (IDR) of hypertension is:

• A 10 per 100 persons
• B 100 per 1000 persons
• C 11.9 per 1000 person-years ✓
• D 8.4 per 1000 person-years

Explanation

Incidence Density Rate = Number of new cases / Total person-time at risk = 500 / 42,000 person-years = 0.01190 per person-year = 11.9 per 1000 person-years. The IDR (also called Incidence Rate or Force of Morbidity) uses person-time as the denominator to account for varying observation periods due to dropouts or late entries. Options A and B express cumulative incidence (attack rate), not incidence density. Option D (8.4) would result from 500/59,500 — incorrect denominator.

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

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