Comparison of control subjects recruited by random digit dialing and area survey

Although random digit dialing (RDD) is an accepted and commonly used method of sampling populations for controls in case-control studies, there have been surprisingly few attempts to compare the accuracy of RDD with that of the best traditional alternative, i.e., household area surveys. The aim of the present study was to compare a variety of characteristics of control subjects selected by RDD and area survey methods. All data were gathered through in-person interviews of both types of control subjects. The area survey identified a population-based sample of 20- to 79-year-old residents of two Washington State counties in 1978 and 1979. Control groups for three case-control studies of bladder cancer, gynecologic cancers, and multiple myeloma were drawn from this area sample, for a total of 240 control subjects. Controls aged 21-64 years from the same counties were identified for the National Bladder Cancer Study using RDD telephone sampling during the same time period. There were 134 respondents in the RDD control group. Overall, the two control groups selected by these two different methods yielded similar estimated frequencies of various population characteristics. The small differences observed in some age/sex subgroups and the statistical significance of the overall measure of association for occupational exposure to organic substances may be attributed to multiple comparisons.