Objective. To determine the factors affecting whether Medigap owners switch to Medicare managed care plans.
Data Sources. The primary data were the 1993–1996 Medicare Current Beneficiary Survey (MCBS) Cost and Use Files. These were supplemented by data available from the Centers for Medicare & Medicaid Services (CMS) website.
Study Design. Individuals on the MCBS files with Medigap coverage in the period 1993–1996 were included in the study. The person-year was the unit of analysis. We used multivariate logistic regression analysis to determine whether or not a Medigap owner switched to a Medicare-managed care plan during a particular year. Independent variables included measures of affordability, need for services, health insurance benefits, sociodemographics, and supply of managed care plans.
Principal Findings. We did not detect strong evidence that beneficiaries in poorer health were more likely than others to switch from Medigap coverage to Medicare-managed care. In addition, higher Medigap premiums did not appear to induce beneficiaries to switch into managed care.
Conclusions. We examined selection bias in joining managed care plans among the subset of Medicare beneficiaries who have Medigap policies. No strong evidence of selection bias was found in this population. We conclude that there was no evidence that the Medigap market is becoming prohibitively expensive as a result of unfavorable selection.