Epidemiology develops measurements that allow to quantify the occurrence of disease within the population. There are three types of measurements: frequency measurements, explained in this article; association measurements, between the occurrence of disease and some characteristics, the effect of which on the disease is what they intend to measure; and measurements of the potential impact that modification or disappearance of some risk factors would have on the occurrence of disease in the population. The first objective of epidemiological studies is the knowledge of the frequency of disease. There are three basic measurements of frequency of a disease. Prevalence measures the proportion of people that has it in a given moment. Cumulative incidence measures the proportion of people that convert from non-sick individual to sick individual during a specified period of time. Incidence rate is a measure of the instantaneous strength of occurrence of the disease.