Cumulative incidence is calculated by dividing the number of new cases of a disease by the number of individuals at risk at the start of the study period. The formula is:
\[ \text{Cumulative Incidence} = \frac{\text{Number of New Cases}}{\text{Population at Risk}} \]
For example, if there are 50 new cases of an illness in a population of 1,000 people over one year, the cumulative incidence would be 0.05 or 5%.
