Predictor variables were those prominent in published research on survey participation and retention, including 7 core demographic variables and 10 physical health variables from either the baseline telephone interview or the SAQ (see Table 1 ). The demographic variables represented baseline characteristics of age (continuous), gender (0 = male, 1 = female), marital status (0 = not married, 1 = married), race (0 = minority, 1 = White),2 educational level (1 = less than high school, 2 = high school graduate or equivalent, 3 = some college, 4 = college graduate or more), personal income (continuous), and county size (1 = 21 largest MSAs [metropolitan statistical areas], 2 = MSA greater than 85,000 households, 3 = MSA greater than 20,000 households, 4 = remaining counties). Among the health variables included were common measures such as subjective physical health rating (1 = poor, 2 = fair, 3 = good, 4 = very good, 5 = excellent), a count of the number of chronic conditions, a count of functional limitations (instrumental activities of daily living [IADL]), and body mass index (BMI; continuous). Two health behaviors, drinking (0 = ever drink 3+ days/week, 1 = never drank 3+ days/week) and smoking history (1 = currently smoke, 2 = smoked in past, 3 = never smoked), were also included as well as health insurance coverage (1 = no coverage, 2 = government coverage, 3 = private coverage), a count of the number of physician visits in the prior year, and a novel measure of alternative medicine/therapy use (respondents were asked to choose from among 13 listed medicines/therapies used in the prior year: 0 = did not use any in prior year, 1 = used at least one in prior year). The latter was viewed as a possible index of “topic interest” in matters related to a survey about health.
Most sociodemographic variables (except income) were obtained from the baseline phone interview, whereas most health questions (except subjective health, smoking, and drinking behaviors) were obtained from the SAQ. The latter was completed by 87% of the RDD respondents and 92% of the twin and sibling respondents. Thus, the present analysis is limited to those individuals with complete data across the predictors and who participated in both waves of the phone survey. Under these criteria, the sample sizes were as follows—RDD: n = 3,140; Twin: n = 1,520;3 Sibling: n = 745.
Most sociodemographic variables (except income) were obtained from the baseline phone interview, whereas most health questions (except subjective health, smoking, and drinking behaviors) were obtained from the SAQ. The latter was completed by 87% of the RDD respondents and 92% of the twin and sibling respondents. Thus, the present analysis is limited to those individuals with complete data across the predictors and who participated in both waves of the phone survey. Under these criteria, the sample sizes were as follows—RDD: n = 3,140; Twin: n = 1,520;
