Validity of random-digit-dialing in recruiting controls in a case-control study.

Objectives: To evaluate whether the participating controls represented the underlying population in a colorectal cancer case-control study in a geographically isolated North American population.

Methods: The characteristics of the study controls recruited through Random-digit-dialing (RDD) were compared with those in the corresponding target population estimates.

Results: Participating controls were more likely to have higher levels of education and income and were less likely to be smokers.

Conclusion: Study controls recruited through RDD tend to have higher socioeconomic status, which may lead to overestimation of a number of risk factors in this study.