Perhaps the most basic definition of a safety net hospital is one that has an "open door" policy to all individuals regardless of their ability to pay. This policy can derive from either a legal mandate to care for charity patients or a mission-driven commitment to such patients. In general, researchers and policymakers have not used this definition alone to identify safety net hospitals. Instead, the legal mandate or mission definition is typically used in combination with other characteristics. For example, in the Clinton health care proposal, safety net hospitals were defined as those hospitals that were legally mandated (mission-driven hospitals were not included) to provide care and were located in areas with high levels of need such as health professional shortage areas (HPSAs). Gaskin and Hadley (1999) classified safety net hospitals as those that have an explicit safety net mission or a high proportion of low-income patients — Medicaid, charity care, or self-pay patients.