Which statement best describes the denominator used in calculating an incidence rate?

• A. Population size at risk at baseline
• B. Sum of different times each individual was at risk (person-time)
• C. Average duration of disease
• D. Number of cases divided by population

Answer: (B)

Incidence rate is about how fast new cases appear over time, so the denominator must reflect time at risk. You sum up the actual amount of time each person was observed and at risk, even if people join later or are followed for different lengths of time. This total is called person-time (for example, person-years). That way, the rate can account for varying follow-up and censoring, giving a precise measure of incidence density. For example, if several people contribute different amounts of follow-up time and one new case occurs, the rate is the number of new cases divided by the total person-time.

Using population size at baseline would estimate a risk (cumulative incidence) over a fixed period, not a rate. The average duration of disease is not the denominator for incidence at all, and simply dividing cases by population ignores how long people were under observation, so it does not yield an incidence rate.