Both incidences and prevalences have a a numerator (the number of cases) and denominator (the size of the sample). However, they differ in an important way. Incidence measures the "rate" at which (new) events occur in the population It may be measured as a true rate (incidence density, incidence rate) or proportion (incidence proportion). In this chapter, we consider incidence proportions. Incidence proportion represent average risks in the population. Prevalence is the cross-sectional likelihood that an individual selected at random from the population will have the condition question. Prevalence is always measured as a proportion.