Evidence-appraisal glossary
Person-time
Person-time is the total amount of time that all participants in a study are observed and at risk for the outcome, added together. It forms the denominator of an incidence rate, so ten people followed for one year and one person followed for ten years both contribute ten person-years.
Also called: person-years, person-time at risk.
Person-time lets a study count exposure to risk fairly when people are followed for different lengths of time or enter and leave at different points. Each person contributes time only while they are being observed and still able to develop the outcome; their clock stops when they get the disease, die, or are lost to follow-up. A common misreading is to treat equal person-time as equal information: ten years from one person and one year each from ten people give the same person-time but are not equally informative, because a single individual's experience cannot show how risk varies across people.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.