The Evaluation of Matching in a Case-Control Study of Colorectal Cancer Using General Practice Lists

Abstract

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.