the evaluation of matching in a case-control study of colorectal cancer using general practice lists

background: a crucial part of a case-control study is the selection of a sample of controls that represent the base-popula­tion from which cases were drawn. controls may be matched to cases by one or more potentially important confounding vari­ables, such as socioeconomic status. in the united kingdom, one method for control selection has been based on the pa­tient list of the general practice with whom the cases were registered, which we refer to as gp-matching. we aimed to ex­plore whether gp-matching adequately control for the potential confounding effect of socioeconomic status. methods: the townsend index of deprivation was calculated for different two national census geography levels of elec­toral ward/postcode sector and enumeration district/output area for the three study areas of dundee, leeds and york. con­ditional logistic regression was used to estimate the association of cases with deprivation (based on the townsend index) com­pared with that of matched controls for the two geographical scales. results: at the largest geographical level (electoral ward/postcode sector) there was no evidence of a difference in the dis­tribution of deprivation scores between cases and controls. however, analysis at the smallest level (enumeration dis­trict/output area) showed that, despite gp matching, cases were more likely to live in deprived areas than matched controls.  conclusion: using general practice lists for the selection of controls for controlling the confounding effect of socioeco­nomic status might not be an appropriate method for case-control studies conducted in the united kingdom.