To control for potential confounders or to enhance stratified analysis in observational studies, researchers may choose to match cases and controls or exposed and unexposed subjects on characteristics of interest. If matching is superfluous or erroneous, overmatching may occur. The three main effects of overmatching are a loss of statistical efficiency, introduction of bias, and loss of financial efficiency.


To reduce confounding or to enhance stratified analysis, unexposed subjects in cohort studies or controls in case-control studies may be chosen to be identical or similar to exposed subjects or cases with respect to the distribution of one or more variables. Overmatching, sometimes referred to as overmatching bias, occurs when matching is done incorrectly or unnecessarily leading to reduced efficiency and biased results. Overmatching generally affects case-control ...

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