Which statement differentiates incidence rate from cumulative incidence?

• A. Incidence rate uses person-time in the denominator.
• B. Incidence rate uses person-time; cumulative incidence is the proportion of people who develop disease during a period.
• C. Incidence rate is the proportion of people who develop disease during the period.
• D. Cumulative incidence uses person-time in the denominator.

Answer: (B)

The difference rests on how time is incorporated into measuring new cases. Incidence rate (incidence density) counts how many new cases occur per unit of person-time at risk, so the denominator reflects the actual time each person is observed and at risk. Cumulative incidence, on the other hand, is a simple proportion: the number of new cases during the period divided by the number of people at risk at the start, assuming everyone is followed for the whole interval or losses are negligible. So incidence rate uses person-time in the denominator to express a rate, while cumulative incidence expresses a risk as a proportion over the period. For example, if 5 new cases arise among 100 people followed for 2 years with some varying follow-up, the incidence rate would use the total person-years in the denominator, whereas the cumulative incidence would be 5/100 = 5% for that period.