Overmatching refers to the unnecessary or inappropriate use of matching in a cohort or case control study.
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
Matching is a statistical technique which is used to evaluate the effect of a treatment by comparing the treated and the non-treated units in an observational study or quasi-experiment. The goal of matching is, for every treated unit, to find one (or more) non-treated units with similar observable characteristics against whom the effect of the treatment can be assessed. By matching treated units to similar non-treated units, matching enables a comparison of outcomes among treated and non-treated units to be able to estimate the effect of the treatment without reduced bias due to confounding. The three main effects of overmatching are a loss of statistical efficiency, introduction of bias, and loss of financial efficiency.
Matching the control group by gestation length and/or the number of multiple births when estimating perinatal mortality and weight at birth after in vitro fertilization is overmatching, since IVF itself increases the risk of premature birth and multiple births.