Five new cases of a disease occur during follow-up of a dynamic cohort. Four subjects were followed for 300 days each before developing the disease or being censored, and the remaining participants contributed a total of 700 person-days. The incidence density is:

• A 2.5 per 1000 person-days ✓
• B 5 per 1900 person-days
• C 5 per 1000 person-days
• D 0.5 per 1000 person-days

Explanation

Incidence density divides the number of new cases by the total person-time at risk. Total person-time is 1200 plus 700, equal to 1900 person-days, so the rate is 5 divided by 1900, approximately 2.6 per 1000 person-days, closest to option A. Option C ignores the denominator arithmetic and simply scales the numerator, while option D misplaces the decimal point.

Reference: Gordis Epidemiology, 6th ed.

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