A case-control study is a type of study design used widely, originally developed in epidemiology, although its use has also been advocated for the social sciences. It is a type of observational study in which two existing groups differing in outcome are identified and compared on the basis of some supposed causal attribute. Case-control studies are often used to identify factors that may contribute to a medical condition by comparing subjects who have that condition/disease (the "cases") with patients who do not have the condition/disease but are otherwise similar (the "controls"). They require fewer resources but provide less evidence for causal inference than a randomized controlled trial.