These were developed to provide information more rapidly than when cohorts are followed for prolonged periods of time, using traditional hands-on methods; case-based research is, however, necessarily retrospective. Analysis begins with the characterization of a group of people that already have the disease of interest, the 'cases'. Control subjects are then drawn from a population with exactly the same attributes as that from which cases are selected (often a very difficult task!). The antecedent demographic, therapeutic and environmental factors of both groups are documented, often by a combination of record abstraction and interview. If differences are found in the proportion (rate) for some factor between the two groups, then this becomes suspected as an etiological agent for the disease of interest. This suspicion is strengthened when either the discovered factor corresponds with a predictive hypothesis at the start of the study, or when there is consistent evidence that would support its identification (perhaps a biochemical link between the factor and the disease).
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