Which term describes a measure that expresses the portion of a population affected by a disease or event?

• A. Ratio
• B. Rate
• C. Rates/Proportion
• D. Infectivity

Answer: (C)

When a measure describes the portion of the population that is affected, you’re looking at a proportion. A proportion is a part-whole ratio: the number affected divided by the total population, giving the fraction (often shown as a percentage) of people with the condition. This contrasts with a rate, which includes a time element and describes how quickly something occurs per population at risk. Infectivity is about how easily a disease spreads, not the share of people affected. So the term that best captures the idea of a portion of the population affected is proportion (often discussed alongside rates in broader contexts, but the portion focus points to proportion).