Attack rates are actually risks (or "incidence proportion" or "cumulative incidence"). Attack rates are often expressed as a percentage. An attack rate is not an incidence rate. It is actually a risk (also called incidence proportions), and the time contribution of each individual is not included in the denominator. The denominator consists of the number of individuals present at the beginning of the outbreak, disregarding those who will leave, develop illness, or die. This means that the cases (numerator) are also included in the denominator: it is therefor a true proportion. In outbreaks of short duration, attack rate is a term frequently used instead of risk or incidence proportion. In a foodborne outbreak, we will often refer to "food specific attack rates". In this circumstance, the denominator will consist of the number of individuals who ate a specific food, while the numerator will consists of the number of individuals who ate that food and became ill.