ABSTRACT
A case-control study is an observational epidemiological study where subjects are selected on the basis of whether they have (cases) or do not have (controls) a particular disease. A comparison is made between the proportion of subjects in the case and control groups who have been exposed to the aetiological factor under study. Case-control studies are always retrospective, i.e. they look back in time to determine past exposure in cases and controls (1b).
