Over-matching can be a design fault in case-control studies and may lead to bias. J L Marsh and colleagues describe a case-control study of workers at a nuclear reprocessing plant in which over-matching obscured the relation between radiation exposure and mortality from leukaemia

This paper describes the rationale of a case-control study, explains the process of stratification through matching, and summarises the mechanism of confounding. We carried out a matched case-control study of workers at BNFL (British Nuclear Fuels) to clarify the results of past cohort studies, which had found a significant association between risk of leukaemia and cumulative external radiation dose. Fitted models from the current study contradicted these results. After examining the relation between the matching factors and the dose, we suggest that over-matching is a cause of the contradiction.