What are the effects of Medicare managed care on access and quality (compared to Medicare fee-for-service) for the general population of managed care beneficiaries and vulnerable populations, in particular? Based on previous studies finding that managed care works best for those who know how to work the system, the researchers at the University of Southern California hypothesize that vulnerable populations are more likely to plan than their non-vulnerable equivalents. They will test this hypothesis at both the patient and plan levels, examining the following questions: 1) Do vulnerable populations enrolled in Medicare managed care receive different levels or quality of care than their less vulnerable counterparts? and 2) Do health plan characteristics (e.g. type of ownership, organizational structure, or experience with Medicare risk contracting) influence the level of care vulnerable populations receive? The goal of this study is to provide policymakers with a deep and broad analysis of the experiences of Medicare managed care enrollees. They will also conduct a series of case studies to assess the technical feasibility of adding outpatient, pharmacy, and long-term care data from health plans to the OSPHD database.