Which statement about incidence proportion versus incidence rate is correct?

• A. Incidence proportion is the probability of the event over a specified period; incidence rate is the number of events per person-time; use proportion with fixed follow-up and rate when person-time varies.
• B. Incidence proportion uses person-time; incidence rate uses fixed time window; use rate when follow-up is fixed.
• C. They are the same measure.
• D. Incidence rate is the probability of the event over a fixed period.

Answer: (A)

Distinguishing probability over a defined time from a rate that uses time-at-risk is the key idea. Cumulative incidence (incidence proportion) is the probability that a person without the disease at baseline will develop it during a specified period. It is appropriate when follow-up is fixed and complete, so you can express risk as the proportion of people who become cases in that window. Incidence rate (incidence density) counts new cases per unit of person-time, so it reflects how quickly cases accumulate and handles varying follow-up times among individuals. Because some people may contribute more or less follow-up time, the rate uses the total person-time at risk rather than a single fixed window. Thus the correct statement is that incidence proportion is the probability of the event over a specified period, while incidence rate is the number of events per person-time; use the proportion when follow-up is fixed and the rate when person-time varies. For example, 50 cases among 1000 followed for a year gives a cumulative incidence of 5%, while if follow-up times differ, the incidence rate would be 50 cases per total person-years observed.