In epidemiology, person-years is used to account for varying follow-up times. Which statement best illustrates this concept?

• A. A single person observed for 10 years contributes 10 person-years.
• B. Person-years measure only the number of people in the study.
• C. Person-years require fixed follow-up for all participants.
• D. Person-years are used to measure prevalence.

Answer: (A)

The main concept is that person-years capture time at risk, not just how many people are in the study. It adds up the amount of time each participant contributes while they are being followed and at risk of the event, so differing follow-up lengths don’t distort the measure.

If one person is followed for 10 years, that person contributes 10 person-years. If two people are followed for 5 years each, together they contribute 10 person-years as well. This illustrates how person-years combine time and number of people to create a consistent metric when follow-up times vary.

The idea that person-years only counts the number of people ignores the time component, so it’s not correct. The notion that fixed follow-up for all participants is required is inaccurate—the strength of person-years is precisely handling varying follow-up. And using person-years to measure prevalence is not right because prevalence is a snapshot of how many people have the condition at a specific time, whereas person-years relate to time at risk and incidence.