A person-year is a unit calculated by multiplying the number of people in a study by the time each person spends in the study. For example, if there were 1,000 people in a study that lasted 2 years, the study would have collected 2,000 person-years of data.

This unit is useful due to the complexities of clinical studies. Most studies have some participants drop out due to life changes, loss to follow up, side effects, death, or other reasons. The person-year unit takes these factors into account to provide a study period based on the specific study population. For example, consider a study with only two participants, A and B. If person A participated in the study for 4 years, and person B participated for 8 years, the total person-years for the study would be 12 person-years. This is calculated by adding the number of years both people were in the study. Person-years are useful not only to quantify how much data was actually collected, but also to calculate incidence rates and some forms of prevalence.^[1]

Incidence rates and prevalence calculations are to measure how often an outcome happens in a study. For example, if some participants experience a particular side effect, person-years will be used when calculating the incidence to determine the rate at which the general population would be likely to experience this side effect. For example, suppose that in a study with a total of 1,000 person-years, there were 30 cases of rash. To find the incidence rate of an outcome, the number of cases of that outcome should be divided by the total person-years. With 30 cases of rash over the study period of 1,000 person-years, the incidence rate of rashes in the study would be 0.03 per person-year. To find the incidence rate as a percentage, you would multiply the incidence rate by 100, giving 3%. Thus, we can say that there were 0.03 cases of rash for every person-year of observation in the population during the study period: an incidence rate of 3%.^[1]