Which statement is true about incidence rate calculations?

• A. It uses the number of new cases divided by the population at risk at the start.
• B. It uses the number of existing cases at a point in time.
• C. It uses the number of new cases divided by person-time at risk.
• D. It is the same as prevalence.

Answer: (C)

Incidence rate measures how fast new cases occur, taking into account the actual time people are at risk. The key is using person-time in the denominator, because people may be followed for different lengths of time or may enter and leave the study. By dividing the number of new cases by the total person-time at risk (for example, person-years), you get a rate that reflects the speed of disease occurrence in the population.

For example, if 5 new cases occur during 100 person-years of observation, the incidence rate is 5 per 100 person-years (0.05 per person-year). This differs from using a simple population-at-risk count, which yields a cumulative incidence proportion (a proportion of people who develop disease over the period). It also differs from prevalence, which counts existing cases at a point in time rather than new cases over time.