What distinguishes cumulative incidence from incidence?

• A. It assumes all persons in the denominator are at risk for the same period
• B. It uses only prevalent cases
• C. It excludes time
• D. It is simply the total number of cases

Answer: (A)

Cumulative incidence is the probability that a person who is at risk at the start of a defined period will develop the disease during that period. It is calculated as the number of new cases during the period divided by the number of people at risk at the beginning. This approach inherently assumes everyone in the denominator is followed for the entire same period, so each individual contributes the same amount of time at risk.

This is why the statement that differentiates cumulative incidence is about the fixed at-risk denominator for the same follow-up interval. It’s not about using prevalent cases, it’s not about excluding time, and it isn’t simply the total number of cases—the value is a proportion, reflecting risk over the defined period.