Introduction: Comparative studies are intended to show possible differences between two or more groups; In addition to surveys that are cross-sectional, as seen in previous discussions, data for comparative studies may come from different sources; the two fundamental designs being retrospective and prospective. Retrospective studies gather past data from selected cases and controls to determine differences, if any, in exposure to a suspected risk factor. These are commonly referred to as case–control studies; each study being focused on a particular disease. [RETROSPECTIVE STUDIES] [CROSS-SECTIONAL STUDIES]
In a typical case–control study, cases of a speciļ¬c disease are ascertained as they arise from population-based registers or lists of hospital admissions, and controls are sampled either as disease-free persons from the population at risk or as hospitalized patients having a diagnosis other than the one under study. [CASE-CONTROL STUDIES]
The advantages of a retrospective study are that it is economical and provides answers to research questions relatively quickly because the cases are already available. Major limitations are due to the inaccuracy of the exposure histories and uncertainty about the appropriateness of the control sample; these problems sometimes hinder retrospective studies and make them less preferred than prospective ones.
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