Evaluation of Random Digit Dialing as a Method of Control Selection in Case–Control Studies

Control groups selected by random digit dialing are frequently used in case–control studies. Concern about the potential for bias in these control groups has been expressed, primarily because of low response rates. This study compares the characteristics of a hypothetical control group consisting of 341 men and women aged 40–74 years, selected by random digit dialing and participating in an interview in 1990, with the characteristics of 15,563 men and women aged 40–74 years who participated in a privately conducted census in the same upstate New York county in 1989. For most measures, no differences were seen between the random digit dialing sample and the census population. However, the hypothetical control group was more likely to have had their cholesterol checked in the past 2 years and was somewhat more likely to have had other screening tests as well. In addition, the hypothetical control group was somewhat better educated. The results suggest that, at least in this setting, control groups selected by random digit dialing are representative of the general population in most respects; however, caution should be used when studying the relation between screening tests and disease occurrence by means of case–control studies using controls selected by random digit dialing. Am J Epidemiol 1992;135:210–22.