In the experimental studies, the collection of individuals to which the experimental process of interest is not applied. In the observational studies, most often used for a collection of individuals not subjected to the risk factor under the investigation. In various medical studies the controls are drawn from the same clinical source as the cases to ensure that they represent the same catchment population and are subject to similar selective factors. These would be termed as, the hospital controls. An alternative is to use control measures taken from the population from which the cases are drawn. The latter is suitable if the source population is well de?ned and the cases are representative of cases in this population.