What is cumulative incidence?

• A. Proportion of a population that develops disease over a specified period.
• B. Average time to occurrence of disease.
• C. Total number of new cases in a given time.
• D. Incidence rate per person-year.

Answer: (A)

Cumulative incidence is the probability that a person who is disease-free at the start will develop the disease during a specified period. It’s expressed as the proportion of the at-risk population that becomes a new case over that time, essentially a measure of risk for that defined interval. You calculate it by taking the number of new cases during the period and dividing by the number of people at risk at the beginning of the period.

This differs from simply counting new cases, which is just a total number and doesn’t tell you how likely it is for someone in the population to get the disease. It also differs from the incidence rate, which uses person-time in the denominator and accounts for varying follow-up times. Average time to disease describes a time-to-event measure, not a proportion of people affected.