Evidence-appraisal glossary
Cumulative incidence
Cumulative incidence is the proportion of a disease-free group that develops the outcome over a fixed follow-up period. You divide new cases by the number of people at risk at the start. It estimates an individual's average risk and always falls between 0 and 100 percent.
Also called: cumulative incidence proportion, incidence proportion, attack rate.
What it is
Cumulative incidence answers "out of everyone who started free of the outcome, what fraction developed it by the end?" The denominator is the count of at-risk people at baseline; the numerator is the new cases during follow-up. Unlike an incidence rate (which uses person-time and can exceed 1), it is a probability bounded between 0 and 1.
How to use it when reading a study
- Check the time window. A cumulative incidence is meaningless without it: 10 percent over 1 year differs sharply from 10 percent over 10 years.
- Watch the denominator assumption. Simple division assumes everyone is followed the full period. When people drop out or die of other causes, honest studies use survival methods (1 minus Kaplan-Meier) or a competing-risks cumulative incidence function instead.
- Compare groups fairly. Absolute risk, risk difference, and number needed to treat are all built from cumulative incidence, so confirm the follow-up length matches across arms before trusting any contrast.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.