a case-control study design is a observation study design which assess the risk of particular outcome (a trait or a disease) associated with an event (either an exposure or endogenous factor). A case-control study design therefore declares an exposure variable which is dichotomous in nature (exposed/non-exposed) and an outcome variable, which is also dichotomous (case or control), thus giving the name to the design. During the execution of the design, a case control study defines a population and counts the events to determine their frequency.