Bias is any systematic error in the design, conduct, analysis, or interpretation of a study that tends to produce an incorrect assessment of the nature of the association between an exposure or risk factor and the occurrence of disease. It is evident that a major emphasis in an epidemiologic study should be to minimize any biases that may occur at any stage of the study. Since by definition, biases will produce an incorrect assessment of the nature and magnitude of an association, avoidance of bias will be particularly important when the association under investigation is already presumed to be weak. The major biases important for evaluating weak associations are described in the context of how they may occur and how they may be avoided at each stage of an epidemiologic study.
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