What is meant by 'person-time at risk' in cohort studies?

• A. The total time that each subject is observed and at risk of the outcome.
• B. The calendar duration of the study regardless of participation.
• C. The time from exposure to outcome for cases only.
• D. The time only during which participants are unexposed.

Answer: (A)

The main idea is that person-time at risk tracks how much observation time each participant contributes while they could still develop the outcome. In cohort studies, incidence is expressed as the number of new cases per unit of person-time at risk, which wins over simply counting people because follow-up times vary and some participants are censored or develop the outcome at different times.

So, person-time at risk is the total time that each subject is observed and at risk of the outcome, summed across all participants. If someone starts disease-free and is followed until they develop the disease, are lost to follow-up, or the study ends, their time contributes to that total up to the point of censoring or event. For example, 100 people followed up to 3 years with some developing the outcome earlier will contribute less than 300 person-years if events happen before the end, reflecting the actual observation time. This concept does not simply measure the calendar length of the study, nor does it track only exposure-to-outcome time for cases, nor only the unexposed period. It captures the precise amount of time people are at risk, across the whole cohort.