Random-digit dialing for control selection in childhood cancer studies: the geographic proximity and demographics within matched sets.

The purpose of this analysis was to evaluate the degree of matching in 95 individually matched pairs from a case-control study of childhood leukemia that used random-digit dialing to select control subjects. Both geographic proximity (of each case subject to his or her matched control subject) and differences in socioeconomic status were evaluated. The median distance between matched pairs was 3.2 km. There were no significant differences in distance between matched pairs by urban/rural status and geographic location. For studies of childhood cancer drawn from pediatric referral centers, random-digit dialing appears to provide a suitable control group.