Incidence rate is often expressed per which unit of time?

• A. Often expressed per 1,000 or 100,000 person-years.
• B. Always per year of calendar time.
• C. Per number of participants at baseline.
• D. Per exposure unit.

Answer: (A)

Incidence rate is about how fast new cases occur and uses the amount of time people are at risk as the denominator. Because people contribute different amounts of follow-up time, we express this rate as the number of new cases divided by the total person-time, typically standardized to 1,000 or 100,000 person-years. This standardization lets rates be compared across studies with varying follow-up lengths. For example, 10 new cases over 2,000 person-years equals 5 per 1,000 person-years. It’s different from simply counting people or using calendar time, and it’s not usually tied to a fixed exposure unit unless the study specifically defines exposure-time as the denominator.