Which measure accounts for new cases only during a period?

• A. Prevalence Rate
• B. Mortality Rate
• C. Case Fatality Rate
• D. Incidence Rate

Answer: (D)

The main idea is distinguishing how disease frequency is measured over time: some measures reflect new occurrences, others reflect all existing cases, and others reflect outcomes. Incidence rate specifically tracks new cases that arise during a defined period among people who are at risk, and it is expressed with a time component (for example, per 100,000 person-years). This makes it a direct measure of the risk of developing the disease during that period.

Prevalence looks at the total number of existing cases at a point in time or over a period, including people who already have the disease, people who have recovered, and even those who may have died; it does not isolate new cases. Mortality rate counts deaths in the population during the period, regardless of disease onset. Case fatality rate looks at the proportion of people with the disease who die from it, among those who have the disease, rather than new cases in the population.

So, when the question asks which measure accounts for new cases only during a period, incidence rate is the best answer. For example, if 50 new cases occur in a population over one year, incidence rate captures those new cases relative to the time people were at risk, reflecting the risk of developing the disease during that year.