Define cumulative incidence.

• A. Cumulative incidence is the probability of developing the outcome over a fixed period in a cohort.
• B. Cumulative incidence is the rate at which new cases occur per unit time.
• C. Cumulative incidence is the number of new cases per person-time.
• D. Cumulative incidence is the difference in risk between exposed and unexposed.

Answer: (A)

Cumulative incidence is the probability that someone who is free of the disease at the start will develop it during a defined period of follow-up in a cohort. It’s a measure of risk expressed as a proportion: number of new cases during the period divided by the number at risk at the start. This makes it a fixed-time, cumulative measure of risk, not a rate.

It’s different from the incidence rate, which counts new cases per unit of person-time and can handle varying follow-up times. It’s also not the same as a difference in risk between groups, which is about comparing risks (an effect measure) rather than defining what cumulative incidence itself is. For example, if 50 of 1,000 disease-free individuals develop the outcome over one year, the cumulative incidence is 50/1000 = 0.05 (5%).