Which statement is true about cumulative incidence?

• A. The proportion of all individuals in the population who have ever developed disease by the end.
• B. Cumulative incidence is a rate per person-time.
• C. Cumulative incidence is the proportion of initially disease-free individuals who develop disease during a defined period.
• D. Cumulative incidence equals period prevalence.

Answer: (C)

Cumulative incidence measures the risk that a person will develop a disease over a defined period, starting from a disease-free baseline. It is calculated as the number of new cases during the period divided by the number of individuals at risk at the start of the period, yielding a proportion (no time units). This focuses on those who begin without disease and tracks how many of them actually develop it over the specified time.

This is different from a rate, which uses person-time in the denominator and describes the speed of occurrence. It’s also different from period prevalence, which counts all existing cases (both new and preexisting) at any point during the interval, rather than the new cases among those at risk at the start. So the statement that best captures cumulative incidence is: it’s the proportion of initially disease-free individuals who develop disease during a defined period. For example, in a disease-free cohort of 1,000 people followed for one year, if 50 develop the disease, the cumulative incidence is 50/1,000 = 0.05, or 5%.