What measurement describes the number of new events that occur during a specified period of time in a population at risk?

The correct answer is that incidence specifically measures the number of new events, such as cases of a disease, that occur in a defined population over a specified period of time. This measurement is crucial for understanding the dynamics of a disease within a population, as it captures the rate at which new cases arise, helping public health officials and researchers track outbreaks or changes in health trends.

In contrast, prevalence refers to the total number of cases of a disease (both new and existing) in a population at a specific point in time, rather than just new occurrences. Morbidity is a broader term that refers to the state of being diseased or the incidence of disease within a population, but it does not specify new cases over time like incidence does. The risk ratio is a measure used in cohort studies to compare the risk of a particular event occurring in two different groups, rather than measuring the number of new events directly.

Incidence provides a clearer picture of how quickly a disease is spreading in a population, making it an essential metric in epidemiology and public health planning.