Incidence rates and prevalence proportions are used to describe the frequency of diseases and health events in populations. They are also used to estimate an association between putative determinants, exposures, and a disease. Epidemiologists often use the term exposures to describe a broad range of events, such as stress, exposures to air pollution or occupational factors, habits of life (such as smoking), social conditions (such as income), or static conditions (such as genetic factors). The term, exposure, is thus used to describe all possible determinants of diseases. We are interested in estimating if, and if so, how strongly these exposures are associated with a disease (increase and decrease). We do that by comparing disease frequencies in exposed and unexposed people.