The basic incidence rate (sometimes called just incidence) is a
measure of the frequency with which a disease occurs in a population over a period of time. The formula
for calculating an incidence rate is:
The numerator (x)
should include only new cases of the disease that occurred during the specified period.
should not include cases that occurred or were diagnosed earlier.
This is very important when working with chronic infectious diseases such as tuberculosis, malaria and HIV.
The denominator (y) is the population at risk.
This means that the people included in the denominator should be able to develop the disease in question during the time period covered. In practice, we usually use census data for the denominator.
The denominator should also represent the population from which the cases in the numerator arose. The population may be defined by geographic area (e.g., St. Francois County) or by membership in a specific group (e.g., employee of Company X, student at School Y). If we are studying a specific group such as students in a school or residents in a long term care facility, we should use a census of that population for an exact denominator.