Cohort studies often aim to estimate disease occurrence by cumulative incidence or incidence rates.

An important component of calculating the incidence rate is the calculation of person-time. For each person in the study, the time they contribute is the time from study enrollment until becoming a case, or the time until study completion or dropping out of the study.  

The incidence rate is the number of persons who newly experience the outcome during a specified period of time divided by the sum of the time that each member of the population is at risk.

Since all people in the cohort study are non-cases at the start, if there is a specific exposure of interest, the relative risk of becoming a case can be calculated for the exposed versus non-exposed.  More details regarding analysis methods will be seen later in the course.