In epidemiology how do I determine what "per number" to use. For example, here is my most recent assignment and the answer but I don't know how to determine if I should use per 100,000 or 1,000 or 10,000.

1. In the Atlanta metropolitan area, 723 new cases of invasive cancer of the cervix occurred among white women between 2012 and 2021, inclusive. An estimated 620,000 white women lived in this area on average during this time period.What is the average annual incidence rate of invasive cervical cancer in this population?

IR = # of new cases of a disease during a given time period / total population at risk

723/620,000 = 0.0011 for 10 years.

To calculate the rate for each year, divide by 10

= (0.0011/10)*100,000 = 11.7 women per 100,000 will get cervical cancer each year