The hospital's census region was obtained from the AHA Annual Survey of Hospitals. Census region is defined by the U.S. Census Bureau.

This is an important stratifier because practice patterns have been shown to vary substantially by region. For example, lengths of stay tend to be longer in East Coast hospitals than in West Coast hospitals.

The HCUP variable name for the hospital's census region has changed over time. In the 1988-1992 NIS, the hospital's bedsize category was stored in the variable ST_REG. In the 1993-1997 NIS, this same information is stored in the variable H_REGION. Beginning with the 1998 NIS, the name HOSP_REGION is used.

The Midwest region was referred to as "North Central" in the 1988-1992 NIS.

For detailed information about the NIS sampling design, see the year-specific report on the Design of the HCUP Nationwide Inpatient Sample.