3. Incidence

The two ways of measuring incidence, incidence proportion (cumulative incidence) and incidence rate, both share the same numerator: the number of new cases that developed over a period of observation.
They are different in how they address the dimension of time:

• Incidence proportion (cumulative incidence) is the proportion of people who develop the outcome of interest during a specified period of time.

• Incidence rate is a true rate whose denominator is the total of the group's individual times "at risk" (person-time).

Depending on how incidence is expressed, two different equations will be employed.