Do Cases and Controls Matched on the First Eight Digits of Their Telephone Number Share Geographic Proximity and Socioeconomic Characteristics?

Purpose The aim of this study is to assess the geographic proximity of cases and controls who share the same “stem” of their telephone number (area code, prefix, and next two digits) and compare their socioeconomic characteristics. Methods We compared residential proximity and characteristics of case–control sets selected in this way with sets consisting of the same cases matched to different controls on all variables except telephone number stem. Results Mean distance between telephone stem–matched pairs was much less than that between pairs not matched by telephone number stem (3.53 and 20.14 km, respectively). There was slight agreement between cases and controls for some measures of socioeconomic status in both sets. We also compared distances between residences of control respondents who shared the same telephone number stem with those between residences of respondents who shared only the same telephone area code and prefix. Mean distances for the two groups were similar. Conclusions Geographic proximity between cases and controls can be achieved by matching on either telephone number stem or only telephone area code and prefix. Adjustment for socioeconomic factors may be needed even if cases and controls live close to each other.