Summary
This section emphasizes the importance of using person-time as the denominator when follow-up durations differ across subjects. Cumulative incidence is a proportion assuming equal follow-up, but in many prospective studies, each subject contributes a variable amount of time. Therefore, incidence rates are calculated by dividing the number of cases by total person-time, and comparisons between groups require methods (such as Poisson regression or adjusted chi-square tests) that properly account for the differing amounts of time at risk. The accurate measurement and comparison of incidence rates enable epidemiologists to better understand relationships between exposures (such as OC use) and outcomes (e.g., breast cancer).