This paper describes the results of the fourth phase of a research project led by University of Minnesota’s State Health Access Data Assistance Center (SHADAC), Centers for Medicare and Medicaid Services (CMS), Assistant Secretary for Planning and Evaluation (ASPE), National Center for Health Statistics (NCHS), and the U.S. Census Bureau. The project is designed to explain why survey estimates of enrollees in Medicaid are lower than the number of enrollees reported in state and national administrative data.