Which statement best defines incidence and prevalence and their calculations in a defined population over a specified time period?

• A. Incidence is the total number of existing cases; Prevalence is the number of new cases during the period.
• B. Incidence is the number of new cases during the period divided by the population at risk; Prevalence is the total number of existing cases divided by the total population.
• C. Incidence is the number of new cases divided by the total population; Prevalence is the number of existing cases divided by the population at risk.
• D. Incidence is the number of new cases divided by the population at risk during the period; Prevalence is the proportion of the population that has ever had the disease.

Answer: (B)

Incidence and prevalence are two ways of describing how a disease affects a population, but they measure different things. Incidence looks at risk: how many people newly develop the disease during a defined period relative to those who could potentially develop it (the population at risk). Prevalence looks at burden at a moment in time: how many people in the population currently have the disease, divided by the total population.

So incidence uses the number of new cases during the period divided by the population at risk, and prevalence uses the total number of existing cases divided by the total population. This matches the chosen definition.

The other options mix up new versus existing cases or use inappropriate denominators (for example, treating prevalence as new cases divided by the population at risk, or incidence as existing cases divided by the population).