Cumulative incidence is defined over a given interval of time as the number of new cases divided by the total number of the cohort observed at the start of the observation interval.

Stated otherwise, CI is the proportion of those who become cases among those who were healthy when they entered the given time interval.

CI can also be defined as the number of new cases as a proportion of the susceptible population at the start of the observation.

CI is a probability and is a measure of risk.

COMPARISON OF INCIDENCE RATE AND CUMULATIVE INCIDENCE

The cumulative incidence (CI) is based on the incidence rate (IR)

CI  is not reliable unless follow-up is for a very short period of time.

When CI is low <0.1, CI is approximately equal to IR.

IR is superior to CI in that competing causes of death operate on both the numerator and denominator of IR but only on the numerator of CI.

IR is suitable for study of dynamic populations.

CI is suited to study of acute diseases with restricted risk periods. It is also suitable for study of fixed cohorts.