In addition to questioning physicians about various aspects of their practice experience, the CTS physician survey collected extensive information about physician attributes and practice characteristics. Covariates in our analyses included the physician's sex, number of years in practice, specialty (family or general practice, internal medicine, medical subspecialties, and surgical specialties), board certification status, and income relative to the median income in the county; whether the physician was a U.S. medical school graduate; and practice setting (1- or 2-physician practice, single-specialty group practice of 3 physicians or more, multispecialty practice, group or staff health maintenance organization, medical school–based practice, hospital-based practice, or other). We also examined the role of managed care within the practice as measured by the percentage of practice revenue paid on a capitated basis, the number of managed care contracts (fewer than 10 vs. 10 or more), and the percentage of patients for whom the physician served as a gatekeeper. Less than 1% of respondents had any missing covariates (item nonresponse <1% for 2 covariates and otherwise nil). Survey data also allowed us to construct proxy patient-level demographic variables (that is, percentage of revenue from Medicare as a proxy for patient age and percentage of revenue from Medicaid as a proxy for patient socioeconomic status). For the subset of physicians in hospital referral regions that contained at least 3 physicians (10 487 out of 10 577 respondents), we were able to construct regional measures of managed care penetration; as was done in earlier work that used these data (14), we calculated the weighted average of the individual reports of the percentage of practice revenue from managed care by survey respondents in the hospital referral region.