Outbreak measures often report the proportion ill among those at risk during a defined period. Which term describes this measure?

• A. Incidence rate
• B. Prevalence
• C. Attack rate
• D. Mortality rate

Answer: (C)

In outbreak investigations, the measure you’re describing is the attack rate. It captures the proportion of people who become ill within a defined period among those who were at risk at the start of that period. You compute it by dividing the number of new cases in the group by the number at risk in that group during the same window. This makes it especially useful for assessing risk linked to a specific exposure and for quick comparisons between exposed and unexposed groups within a short time frame. It’s different from incidence rate, which uses person-time in the denominator and expresses a rate rather than a simple proportion; prevalence describes the proportion ill at a single point in time or over a period, not necessarily in a defined risk window for new illness; and mortality rate focuses on deaths rather than all illness.