In a study with 100 patients, 25 of whom participated for 2 years, and the other 75 who participated for 6 years, the denominator of the incidence rate as the total number of patient-years exposed is 500 patient-years [(25 patients x 2 years) + (75 patients x 6 years) = 50 + 450 patient-years].
If among those 100 patients, 5 occurrences of accidental bone fracture were observed, then the risk (proportion) would have been 5/100 patients or 5%.
We assume that the risk during the remaining 4 years for the 25 patients with 2-year participation would be no different from their first 2 years or from the 6 years of participation of the other 75 patients.
However, the incidence rate of accidental bone fracture would have been 5 per 500 patient-years, or 1/100 patient-years. This means that, on average, there was 1 occurrence of accidental bone fracture per 100 patients, over the course of each 1 year of treatment.
Incidence rates are used in many epidemiological studies and statistical assessments of risk which allows researchers to reveal trends and communicate levels of risk.
Studies of new medicines can express their safety findings using incidence rates based on 100 patient-years.1