Person-years is a term researchers often use to describe the incidence of a medical event, meaning the rate of new cases or events over a specified period for the population at risk for the cases or events. In medical research, investigators will define the incidence of newly identified cases of a disease or condition in a population at risk over a specified timeframe.
Most often, the statistic researchers will use is 100,000 person-years, meaning they have assessed the health of 100,000 people for one year, according to this article on “incidence” from StatPearls. An example of incidence would be 795,000 new strokes in the United States, annually, meaning the incidence is 795,000 new strokes, among the population in the United States, and the timeframe would be one year.