This chapter reviews the basic types of epidemiological measures, including disease frequency and association, and relates these to the study design. Prevalence is the count of all cases of a disease or condition over some time period and can be presented either at a specific point in time or point prevalence or during a period of time or period prevalence. The cumulative incidence proportion is calculated as disease count divided by at-risk population. The basis of the cross-sectional study is a measure of prevalence; therefore, the measure of association is the prevalence ratio. As the basis of the cohort study is a measure of incidence, the measure of association is a ratio of the incidences between the exposed and unexposed groups. Incidence cannot be directly estimated in a case control study, because the researcher fixes the incidence at a predetermined level due to the ratio of case and control sampling.