The denominator is the number of people at risk in the population during a specified time period. In order for incidence rate to be meaningful, all those included in the denominator must have the potential to develop the condition of interest (that is, become a case). For instance, if we are calculating the incidence of appendicitis, the denominator must include only those without history of appendectomy. Similarly, when calculating the incidence of prostate cancer, the denominator should exclude women and those who have undergone prostatectomy.
Even when we know who all are at risk in a population, it is not always possible to obtain observations from all such persons for the entire duration under consideration. Therefore, we have two situations regarding observation of those at risk during a specified time period:
- All at risk people are observed throughout the specified time period
- All at risk people are not observed for the entire time period
When all at risk people are observed throughout the specified time period
Regardless of the duration of time- a week, a fortnight, a month, etc.- all should have been followed up for the entire period. More importantly, the time period should be specified clearly, and all should have been at risk for developing the condition.
The incidence calculated in this manner is also called cumulative incidence proportion.