Which formula is used to obtain the incidence rate?

The incidence rate is a crucial measure in epidemiology that quantifies the frequency of new cases of a disease in a specified population during a defined time period. The formula for calculating the incidence rate specifically focuses on new cases, which makes option A the correct one. It considers the number of new cases of a disease that arise over a specified time and divides that by the population at risk during that same time frame.

This approach allows for the assessment of the risk of developing the disease within the population, highlighting the dynamic aspect of disease occurrence and the potential for disease spread in a population that is at risk of developing that same disease.

In contrast, other options involve metrics that either account for total cases rather than new cases, such as the prevalence measure in option C, or focus on averages of observed measurements or preventable incidents. Those calculations do not specifically capture the incidence of new cases in a defined population and time period, which is the primary concern when determining the incidence rate.