Which statement correctly defines incidence rate?

• A. Incidence rate = new cases divided by person-time at risk.
• B. Incidence rate = new cases divided by population at risk over a time period.
• C. Incidence rate = total population at risk divided by time.
• D. Incidence rate = the number of existing cases at a point in time divided by population.

Answer: (A)

Incidence rate measures how fast new cases of a disease appear in a population, and it uses person-time at risk in the denominator. This means you count the number of new cases during the observation period and divide by the total amount of time each person was at risk and under observation. Expressed another way, it accounts for both how many people are observed and for how long they are observed before they develop the disease or are censored.

For example, if 4 new cases occur over 2,000 person-months of observation, the incidence rate is 4 / 2,000 = 0.002 per person-month, or 2 per 1,000 person-months. This approach differs from dividing new cases by the population at risk without considering how long each person was followed, which yields cumulative incidence (risk) rather than incidence rate. It also differs from a prevalence calculation, which uses existing cases at a single point in time divided by the population.

So the correct statement is that incidence rate equals new cases divided by person-time at risk.