Four patients are followed for development of tuberculosis: one for 2 years, one for 3 years, one for 5 years, and one for 10 years. Two of them develop tuberculosis during follow-up. The incidence density of tuberculosis in this group is:

• A 20 per 100 person-years
• B 10 per 100 person-years ✓
• C 50 per 100 person-years
• D 5 per 100 person-years

Explanation

Incidence density divides the number of new cases by total person-time of observation. The four patients contribute 2 plus 3 plus 5 plus 10, equal to 20 person-years. With 2 cases, the rate is 2 per 20 person-years, that is 10 per 100 person-years. Cumulative incidence (2 of 4, or 50 percent) ignores unequal follow-up durations and would give option C, the commonest error in this calculation.

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

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