Which statement defines the attack rate?

Which statement defines the attack rate?

• A. Number of new cases divided by the population at risk
• B. Number of new cases divided by the number of people at risk
• C. Number of people at risk who develop illness divided by the total number of people at risk
• D. Proportion of the population with disease at a given time

Answer: (C)

Attack rate is the proportion of people at risk who develop illness during a defined outbreak. It reflects the probability that a person in the at‑risk group will become ill over a specified period. You calculate it by taking the number of new cases (people who actually develop illness) and dividing by the total number of people at risk. The denominator should include only those who could become ill, excluding those already ill or immune, so you’re measuring risk within the at‑risk population. The statement that uses new illness cases as the numerator and the total at‑risk population as the denominator expresses this precisely. For example, if 20 of 100 at‑risk individuals become ill during the outbreak, the attack rate is 20/100 = 0.20 (20%). Other options either describe prevalence (disease presence at a point in time) or don’t specifically frame the calculation over the at‑risk group.

Attack rate is the proportion of people at risk who develop illness during a defined outbreak. It reflects the probability that a person in the at‑risk group will become ill over a specified period. You calculate it by taking the number of new cases (people who actually develop illness) and dividing by the total number of people at risk. The denominator should include only those who could become ill, excluding those already ill or immune, so you’re measuring risk within the at‑risk population. The statement that uses new illness cases as the numerator and the total at‑risk population as the denominator expresses this precisely. For example, if 20 of 100 at‑risk individuals become ill during the outbreak, the attack rate is 20/100 = 0.20 (20%). Other options either describe prevalence (disease presence at a point in time) or don’t specifically frame the calculation over the at‑risk group.