Which measure reflects the number of new cases that occur in a population over a specified time period relative to the population at risk?

• A. Attack rate
• B. Prevalence
• C. Incidence proportion
• D. Case fatality rate

Answer: (C)

Measuring new disease occurrences over a defined time uses the number of people who develop the disease divided by the number who were at risk at the start. This yields the incidence proportion, also called cumulative incidence, and it answers the question: what proportion of those at risk became cases during the period?

For example, if 1,000 people without the disease are followed for one year and 50 develop it, the incidence proportion is 50 out of 1,000, or 5%. If some people already had the disease at the start, they’re removed from the at-risk group, and you’d use the remaining at-risk population in the denominator.

Prevalence, in contrast, measures existing cases at a point in time or over a period, not new cases, so it reflects how widespread disease is rather than the risk of developing it. Incidence rate (incidence density) uses person-time in the denominator to account for varying follow-up times. Case fatality rate looks at outcomes among those who already have the disease, not at new case occurrence. Attack rate is a term often used for outbreaks or specific exposure scenarios and is similar to cumulative incidence in that context. The measure described is incidence proportion.