The hospital's teaching status was obtained from the AHA Annual Survey of Hospitals. The missions of teaching hospitals differ from nonteaching hospitals. In addition, financial considerations differ between these two hospital groups. Currently, the Medicare DRG payments are uniformly higher to teaching hospitals than to nonteaching hospitals.

A hospital is considered a teaching hospital if it has one or more Accreditation Council for Graduate Medical Education (ACGME) 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. Beginning with the 2014 NIS, there is an increase in the number of hospitals identified as teaching facilities because the AHA Annual Survey showed an increase in facilities with approved residency programs. About this time, the ACGME became the primary organization for residency training approval.

The HCUP variable name and definition for the hospital's teaching status has changed over time. Beginning with the 1998 NIS, the variable name HOSP_TEACH is used, and teaching hospitals include hospitals with a ratio of .25 or higher of full-time equivalent interns and residents to non-nursing home beds. In the 1993-1997 NIS, teaching status is stored in the variable H_TCH and does not include the ratio of interns and residents to beds. In the 1988-1992 NIS, the hospital's teaching status is not available as a separate variable.

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