Cumulative incidence over a specified period is best defined as which of the following?

• A. Cumulative incidence equals the number of new cases during the period divided by the population at risk at the start.
• B. Total cases at the end divided by the population at the start.
• C. The proportion of the population with the disease at a given point in time.
• D. Incidence rate equals the number of new cases divided by person-time at risk.

Answer: (A)

Cumulative incidence captures the risk of developing a disease over a defined period. It is the proportion of people who were free of the disease at the start and go on to develop it during the specified time. The correct formulation uses the number of new cases during the period divided by the population at risk at the beginning (those who do not already have the disease). This distinguishes it from prevalence, which measures how much of the population has the disease at a single point in time, and from incidence rate, which uses person-time in the denominator to account for different follow-up times. For example, if 1,000 disease-free individuals are followed for a year and 50 develop the disease, the cumulative incidence is 50/1000 = 0.05, or 5%. Cumulative incidence assumes complete follow-up or proper handling of losses to follow-up; it does not include people who already have the disease in the denominator.