2.1. Study setting and population

This secondary analysis used data collected from the National Hospital Ambulatory Medical Care Survey (NHMACS) and met criteria for exemption by the Institutional Review Board. NHAMCS is an annual, national probability sample of ambulatory visits to non-federal, general, and short-stay hospitals in the United States (U.S.) [6]. NHAMCS uses a four-stage probability sampling design, collecting a nationally representative sample of ED visits [6]. At each sampled hospital, NHAMCS data are abstracted from patient records by trained hospital staff members monitored by the U.S. Census Bureau’s agents during a randomly assigned 4-week reporting period.