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-controlled 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 control trial. (from Wikipedia)