What is person-time and how is it used to compute incidence rate?

• A. Person-time is the sum of follow-up time across all individuals at risk; incidence rate = new cases divided by total person-time.
• B. Person-time is the number of people in the study; incidence rate = new cases divided by baseline population.
• C. Person-time is the time at which the first case occurs.
• D. Person-time is the time until the last participant is recruited.

Answer: (A)

The key idea is that incidence rate uses person-time to account for how long people are actually at risk and under observation. Person-time is the total amount of time all individuals contribute while they are at risk and being followed. Each person contributes time until they develop the disease, are lost to follow-up, die, or the study ends. The incidence rate is then the number of new cases divided by the total person-time, giving a rate such as cases per person-year.

This approach is especially useful when people enter the study at different times or are followed for different lengths of time, because it standardizes the risk across varying follow-up durations. For example, if you watch several people for different lengths of time and count how many develop the disease, you divide the number of new cases by the sum of everyone’s observation time to get the rate.

Other interpretations mix up counts with time: it’s not just the number of people, not the baseline population, not the time to the first case, and not the time until the last participant is recruited. Those ideas don’t capture the idea that each person’s contribution to the denominator is the time they were actually at risk and observed.