How is the incidence rate calculated from data with person-time?

• A. Number of new cases divided by total person-time at risk.
• B. Incidence rate equals total population divided by number of cases.
• C. Incidence rate equals prevalence divided by time.
• D. Incidence rate equals number of exposures divided by number unexposed.

Answer: (A)

Incidence rate is about how fast new cases occur, taking into account how long each person is actually at risk. With data that include person-time, you add up all the time that each person was at risk during the study to get the total person-time. Then you divide the number of new cases that occurred by that total person-time. The result is a rate, often stated as cases per person-time unit (for example, per 1,000 person-years). This approach correctly handles varying follow-up times and censoring.

So, for example, if there are 5 new cases over 2,000 person-years of observation, the incidence rate is 5 divided by 2,000, or 0.0025 per person-year (which can be written as 2.5 per 1,000 person-years).

This differs from dividing by the total population or by time alone, or from ratios that involve exposures versus unexposed, which do not reflect the rate of new cases per unit time at risk.