In a study calculating the incidence rate of ovarian cancer, which group should be in the denominator?

The denominator in an incidence rate calculation should specifically include all individuals who are at risk of developing the condition of interest during the time period being studied. In the context of calculating the incidence rate of ovarian cancer, the correct group to include in the denominator is all biologically female participants who do not have ovarian cancer at the start of the study. This is essential because these individuals are the ones who are eligible to develop the disease during the study period.

By including only those without ovarian cancer at the outset, the incidence rate can accurately reflect new cases of the disease that emerge, providing a clear picture of how frequently new diagnoses occur within a defined population over a specific time frame. This approach ensures that you are measuring new incidents rather than counting those who already have the disease, which would misrepresent the rate of new cases.

Including all biologically female participants (regardless of cancer status) or only those who have been newly diagnosed would either inflate the potential incidence or fail to capture the appropriate risk group. Similarly, having all biologically female participants with ovarian cancer at the start would not contribute to understanding the incidence of new cases but rather reflect a prevalence perspective, not incidence.