<P> A case - control study 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 . </P> <P> The case - control is a type of epidemiological observational study . An observational study is a study in which subjects are not randomized to the exposed or unexposed groups, rather the subjects are observed in order to determine both their exposure and their outcome status and the exposure status is thus not determined by the researcher . </P> <P> Porta's Dictionary of Epidemiology defines the case - control study as: an observational epidemiological study of persons with the disease (or another outcome variable) of interest and a suitable control group of persons without the disease (comparison group, reference group). The potential relationship of a suspected risk factor or an attribute to the disease is examined by comparing the diseased and nondiseased subjects with regard to how frequently the factor or attribute is present (or, if quantitative, the levels of the attribute) in each of the groups (diseased and nondiseased)." </P>

The case-control study design is very effective for studying rare diseases