The paper describes the results of the first phase of a four-phase research project among the University of Minnesota’s State Health Access Data Assistance Center (SHADAC), the Centers for Medicare and Medicaid Services (CMS), the U.S. Department of Health and Human Services Office of the Assistant Secretary for Planning and Evaluation (ASPE), and the U.S. Census Bureau. The research project is designed to explain why discrepancies exist between Census Bureau survey estimates of enrollment in Medicaid and the number of enrollees reported in state and national administrative data. Project results will benefit the Census Bureau and other participating agencies because they can be used to improve evaluation of the Medicaid programs (e.g., estimating the effects of proposed policy changes) and to improve survey methods used to collect health insurance coverage information. The research project is divided into four phases; please see Appendix I-A for a description of each phase.