The classification of hospitals as both urban/rural and teaching/nonteaching has changed over time.

• Location: Prior to 2004 data, the urban/rural designation was based on Metropolitan Statistical Areas (MSA). A metropolitan statistical area was considered urban, and a non-metropolitan statistical area was rural. Beginning with the 2004 data, this designation was determined by the Core Based Statistical Area (CBSA). Hospitals residing in counties with a CBSA type of metropolitan were considered urban, while hospitals with a CBSA type of micropolitan or non-core were classified as rural. This change in 2004 contributed to a slight decline in the number of hospitals that were classified as rural and a corresponding increase in the number of hospitals that were classified as urban.
• Teaching Status: Beginning in 1998, a hospital is considered to be a teaching hospital if it has an AMA-approved residency program, is a member of the Council of Teaching Hospitals (COTH) or has a ratio of full-time equivalent interns and residents to beds of .25 or higher. Prior to 1998, a hospital is considered to be a teaching hospital if it has an AMA approved residency program or is a member of the COTH.

Rural hospitals were not split according to teaching status, because rural teaching hospitals were rare.

The HCUP variable name for the hospital's location and teaching status has changed over time. In the 1988-1992 NIS, the hospital's location and teaching status was stored in the variable LOCTEACH. In the 1993-1997 NIS, this same information is stored in the variable H_LOCTCH. Beginning with the 1998 NIS, the name HOSP_LOCTEACH is used.

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

HOSP_LOCTEACH is missing if the data source that contributed discharge data to the NIS prohibits the release of hospital identifiers.