At a corporate picnic, 99 people ate potato salad and 30 of them developed gastroenteritis. What is the food-specific attack rate?

1. A food-specific attack rate divides the people who ate a food and fell ill by everyone who ate that food. 2. 30 divided by 99 equals 0.303. 3. Multiplying by 100 gives 30.3%. 4. 69.7% is the complement, the share of potato salad eaters who stayed well. 5. 23.3% comes from dividing 30 by 129, which double counts the ill, and 3.3% comes from inverting the fraction and slipping the decimal point. _Source: CDC Principles of Epidemiology in Public Health Practice (3rd ed., US federal, public domain), Lesson 3 "Measures of Risk", section Definition of incidence proportion_