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Observational studies in clinical research can be classified as either analytic or descriptive (Table 12–1). Analytic observational studies are similar to randomized, controlled clinical trials in that the goal is to estimate the causal effect of an exposure on an outcome. Also similar to trials, analytic observational studies always include some type of comparison group, against which the experience of the exposed group is compared. Well-designed analytic studies can generate strong evidence for or against a stated hypothesis. Descriptive studies, on the other hand, aim to describe the characteristics or experiences of a particular patient group. Even well-designed descriptive studies cannot be used to draw strong conclusions about the effect of an exposure on an outcome. Instead, these studies are often used to generate study questions that can then be tested by more rigorous methods.

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Table 12–1 Comparison of Features of Analytic and Descriptive Study Designs
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Although many observational study designs are available to researchers (1), a few are most widely used and will be described below. The analytic study designs presented are the case-control study and the cohort study. The descriptive study designs presented are the ecologic study, the cross-sectional prevalence survey, and case reports or case series.

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Case-Control Studies

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In a case-control study, the study population is selected based on a person's outcome status (2). For most case-control studies, the outcome is a disease. Cases are those that have or have had ...

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