PROJECT ABSTRACT Frequent users of emergency medical services (EMS), or EMS ?super-users?, present a burden to local EMS systems and the broader health care system by contributing to emergency department (ED) overcrowding and inefficient hospital use. Due to limitations in existing national-level data sources and the high level of heterogeneity in local- and state-level EMS systems, EMS use is an under-researched area of the health services literature. Current knowledge is limited and understood in the context of ED and inpatient utilization. Applying a behavioral-ecological framework, this innovative multi-methods study seeks to provide an improved understanding of the relationship between frequent EMS use and the broader health care system. In doing so, the study team hopes to identify possible context-specific factors that could lead to reductions in frequent EMS use, more appropriate use of health care resources, and lower costs. The central hypothesis is that there is substantial variation in health care resource use, costs, and outpatient network connectivity relating to specific characteristics of the EMS super-user population and longitudinal patterns of frequent EMS use. The study will use Medicaid claims data over a 6-year period (FY2014-19) from the District of Columbia (DC), which is chosen for its high per-capita EMS call volume and low uninsured rate. Individual characteristics, health care utilization, and Medicaid costs for adult EMS users will be assessed using regression analyses of de-identified claims data (Aims 1 and 2). Outpatient provider network connectivity will be explored using a network analysis approach applied to claims-derived shared-patient provider networks (Aim 3). This research is especially timely, since the Affordable Care Act has encouraged the testing of innovative payment and service-delivery models, such as Mobile Integrated Health-Community Paramedicine, alternative destination programs, and collaborations at various EMS agencies nationwide. Study findings are expected to provide actionable, context- specific insights that can guide Medicaid-driven efforts to improve care efficiency for EMS super-users in urban settings. Further, the study will provide a useful example of how disparate methods applied to administrative claims data can be leveraged to promote population health. The study builds on the PI's methods training, research, and professional experience in care coordination for vulnerable populations, and is a collaborative effort between DC?s Medicaid agency and Fire/EMS Department. Supported by the research team?s expertise in health economics, health services research, econometrics, network analysis, as well as experience in working with EMS, hospital, outpatient health care systems, and community partners, the proposed study will help advance the PI's career goal of bridging the research-practice divide as an independent health services researcher.