Does matching introduce confounding or selection bias into the matched case-control design?

The impact of matching on confounding control in case-control studies remains a sub-ject of ongoing debate, with varying perspectives among researchers. While matching is a well-established method for controlling confounding in cohort studies, its effectiveness in mitigating confounding in case-control studies has long been questioned. Recent studies have determined that matching doesn’t eliminate confounding but, instead, introduces a selection bias on top of the initial confounding, as indicated by causal diagram analysis. This conclusion suggests that the control of initial confounding through matching is either only partial or non-existent. However, this conclusion may not be accurate in exactly matched design because causal diagram cannot always reveal precisely the interplay between the initial confounding and the matching induced selection effect. In this paper, we employ analytical results in conjunction with causal diagrams to demonstrate that the cancellation of the initial confounding by the selection effect is complete in exact individually matched case-control studies. Nevertheless, this cancellation results in a residual selection effect that establishes a backdoor connection between the matching factors and the outcome in the matched design. Failure to adjust for this residual selection effect leads to biased estimates of the exposure effect. Furthermore, this backdoor connection causes matching factors to act like confounding factors in the matched case-control design, which complicates the interpretation of the bias introduced by matching in current literature.