Which statement correctly defines the incidence rate?

• A. It measures all existing cases at a single point in time.
• B. It is the total number of cases in the population.
• C. It is a measure of the speed at which new cases occur, per population at risk, over a period.
• D. It is defined as the number of new cases divided by the population at risk during a time period.

Answer: (D)

Incidence rate is about how fast new cases appear in people who are at risk during a defined time. It expresses this speed by relating the number of new cases to the population at risk within that period, showing how quickly the disease is entering the population. That makes the definition “the number of new cases divided by the population at risk during a time period” the best fit, because it ties new events to the at‑risk group over the interval. Remember, prevalence counts all existing cases at a single point in time, which is a different concept, and simply counting total cases without linking to the time period or at-risk population doesn’t measure the rate at which new cases occur. In practice, some precise formulations use person-time, but the given definition still captures the essential idea of the rate of new cases in the at-risk population over the period.