Which measure describes the number of new cases in a population divided by the population at risk during a period?

• A. Incidence rate
• B. Prevalence rate
• C. Mortality rate
• D. Life expectancy

Answer: (A)

Incidence rate describes how often new cases of a disease appear in a population during a specified period, using those who are at risk as the denominator. The idea is to measure the speed at which people who do not yet have the disease develop it over time, often expressed per person-time (for example, per 1,000 person-years). This focuses on new occurrences and the population that could potentially develop the condition.

Prevalence, by contrast, measures the total number of existing cases (both new and long-standing) at a given time relative to the population, and it is influenced by how long people live with the disease. Mortality rate looks at deaths in a period per population, not new cases of a disease. Life expectancy is an average number of years a person is expected to live and does not describe disease incidence.