Alternative strategies for selecting population controls: comparison of random digit dialing and targeted telephone calls.

PURPOSE: This project was carried out to identify a valid framework for selecting controls to be used in a population-based case-control study of breast cancer, and to compare participation rates and characteristics between women contacted using a standard random digit dialing (RDD) strategy and those who were sent a letter of presentation prior to telephone contact (targeted telephone calls, TTC).
METHODS: Twelve hundred women, ages 20-74, were sampled from the Department of Motor Vehicles (DMV) and Health Care Financing Administration (HCFA) records. Women for whom telephone numbers were obtained (N = 771) were randomly assigned to RDD or TTC. The respondents participated in a brief telephone interview. Odd ratios (OR) and their 95% confidence intervals (CI) were used to estimate differences in characteristics of the respondents between the two contact strategies.
RESULTS: Telephone numbers were obtained for 79% of women aged > or = 55 years and for only 38% of women aged < 55 years. Interviews were obtained for 48% of women for whom we obtained telephone numbers, and for 77% of women for whom eligibility was confirmed via telephone contact. Participation of target women appeared to be higher for the TTC than the RDD group (42% vs. 35%, p = 0.054). Among respondents who were > or = 55 years old, those in the TTC group were 80% more likely (OR = 1.8, 95% CI: 0.9-3.4) to report a serious medical condition than women in the RDD group, 60% less likely (OR = 0.4, 95% CI: 0.2-1.0) to report having used oral contraceptives, and 80% less likely (OR = 0.2, 95% CI: 0.1-0.5) to report having had breast surgery.
CONCLUSIONS: Characteristics of respondents differed according to method of contact. These differences, along with the sampling frame used, should be considered when interpreting findings of case-control studies.