Incidence rate provides information on new cases within a population at risk while providing some information about the timing of infection. Incidence rate is not a proportion and thus can range from zero to infinity. For example, consider the following.

You follow a group of 5 people at risk for contracting a deadly bacterial disease for 6 months. You lose Subject 1 subject after two months because of relocation, Subject 2 contracts the virus after three months, Subject 3 contracts the virus after two months but is cured two months after that, Subject 4 dies from other causes after 4 months, and Subject 5 joins the study a month in and never contracts the virus.

How many person-months would be included in the equation? What would the numerator be?



           Figure 2.1 illustrates this example and provides the answers to these questions. Two people in the at-risk population studied developed the disease over the 18 person-months observed. Thus, the incidence rate is 2/18 person-months (read “two per eighteen person-months”), or can be simplified to 1/9 person-months or 11/100 person-months. It is crucial to include the correct unit of time.