Which statement correctly defines incidence rate?

• A. Incidence rate is the probability of developing the outcome over a fixed period.
• B. Incidence rate is the number of new cases per person-time at risk.
• C. Incidence rate is the total number of existing cases in the population.
• D. Incidence rate equals relative risk.

Answer: (B)

Incidence rate is about measuring how fast new cases appear in a population, taking into account both how many people are followed and for how long they are followed. It is calculated as the number of new cases divided by the total time that people were at risk, so the units are typically cases per person-time (for example, cases per 1,000 person-years). This differs from simply the probability of developing the outcome over a fixed period, which is called risk or cumulative incidence and does not account for varying follow-up times. It’s also not the total number of existing cases in the population (prevalence), and it’s not a measure of association like relative risk, which compares incidence between groups. So the statement that defines incidence rate as the number of new cases per person-time at risk is the correct one.