Matched-pair case-control studies when risk factors are correlated within the pairs

Background. if pair members are independent, simple matched-pair case-control studies are known to yield consistent estimates of the population odds ratio. If pair members are not independent, this is not necessarily true, It has been shown previously that the usual matched-pair estimate remains consistent if the exposure of interest is correlated within the pairs. However, the effect of correlation of unmeasured risk factors within the pairs has not been studied. Methods. We examine the effect of within-pair correlation of unmeasured risk factors independent of the measured exposure. This is done within the context of a simple matched-pair case-control study. We compare the large-sample expectation of the usual matched-pair estimate to the population odds ratio. Results. We show that the usual estimate may be inconsistent in the presence of this correlation. However, if the disease is rare, the magnitude of the bias will be negligible. Conclusions. Correlation of unmeasured risk factors independent of the measured exposure is not a practical problem in this setting.