Person-Time: Definition & Formula

Person-Time

Person-time is a denominator that combines the number of subjects under observation with the actual length of time each subject was observed while at risk for the outcome under study, expressed in units such as person-days, person-months, or person-years. It is used specifically as the denominator of an incidence rate (also called a person-time rate) when the length of follow-up differs across subjects -- because they entered the study at different times, left early, were lost to follow-up, died of an unrelated cause, or (once they experience the outcome) stop contributing further person-time for that outcome. A measure qualifies as using person-time, rather than a simple head-count denominator, specifically when subjects contribute unequal amounts of observed at-risk time and that variation is summed rather than ignored: ten subjects each followed for one year contribute the same 10 person-years as one subject followed for ten years, even though the two scenarios describe very different studies.

ByCASRAI Editorial Board

· Last updated 1 Sept 2026

Why a head count alone is not enough

In a dynamic cohort, subjects rarely all enter and exit a study on the same schedule: some enroll late, some are lost to follow-up, some die of an unrelated cause, and everyone who experiences the outcome under study stops contributing further at-risk time for that outcome from that point forward (a form of censoring). Summing each subject’s actual observed at-risk time — rather than treating the denominator as a fixed population size — produces person-time, typically expressed in person-days, person-months, or person-years depending on how quickly the outcome of interest tends to occur.

Formula

Incidence rate (person-time rate) = number of new cases ÷ total person-time at risk, typically expressed per some round base (per 1,000 or per 100,000 person-years).

Key property

Person-time is interchangeable across combinations that sum to the same total: 10 subjects followed for 1 year each contribute the same 10 person-years as 1 subject followed for 10 years. This is what makes person-time rates comparable across cohorts with very different enrollment patterns and follow-up lengths, which a simple case-count-over-population-size calculation cannot do.

Related CASRAI terms

Attack rate — a related but distinct proportion that uses a fixed population at risk over a single bounded exposure window, not accumulated person-time.
Case fatality rate — a case-based proportion, as distinct from the person-time-based incidence and mortality rates this term supports.
Cohort study vs. case-control study — person-time denominators are calculated within cohort designs where follow-up duration varies by subject.
What is epidemiology? — broader context on the discipline and its measures of disease frequency.

Use in your systems

JATS XML <role> element

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Schema.org DefinedTerm (JSON-LD)

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