Which epidemiologic measure counts new cases in a population during a specified period?

Which epidemiologic measure counts new cases in a population during a specified period?

• A. Incidence
• B. Prevalence
• C. Mortality rate
• D. Case rate

Answer: (A)

New disease occurrence over a defined time frame is captured by incidence. Incidence counts only new cases that develop in a population at risk during the specified period, so it reflects the risk of developing the disease. It can be expressed as a cumulative incidence (the proportion of the at‑risk population that becomes diseased over the period) or as an incidence rate using person-time. This differs from prevalence, which counts all existing cases (new and old) at a point or over a period, not just new ones. Mortality rate tracks deaths, not the onset of disease. Case rate isn’t a standard precise term for this concept, so it’s not the best descriptor for counting new cases over time.

New disease occurrence over a defined time frame is captured by incidence. Incidence counts only new cases that develop in a population at risk during the specified period, so it reflects the risk of developing the disease. It can be expressed as a cumulative incidence (the proportion of the at‑risk population that becomes diseased over the period) or as an incidence rate using person-time.

This differs from prevalence, which counts all existing cases (new and old) at a point or over a period, not just new ones. Mortality rate tracks deaths, not the onset of disease. Case rate isn’t a standard precise term for this concept, so it’s not the best descriptor for counting new cases over time.