NIS_STRATUM is a four-digit stratum identifier used to post-stratify hospitals for the calculation of universe and frame weights. Prior to 1998, this data element was named STRATUM.

NIS_STRATUM includes the hospital's census region or division, ownership/control, location/teaching, and bedsize. Information was obtained from the AHA Annual Survey of Hospitals.

• Prior to 2012, the NIS was stratified by census region. Beginning with 2012, the NIS is stratified by census division.
  
  States, by U.S. Census Bureau Region and Census Division
  
  
  • Region 1 (Northeast)
    • Division 1 (New England) Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, Connecticut
    • Division 2 (Mid Atlantic) New York, Pennsylvania, New Jersey
  • Region 2 (Midwest) (Prior to June 1984, the Midwest Region was designated as the North Central Region)
    • Division 3 (East North Central) Wisconsin, Michigan, Illinois, Indiana, Ohio
    • Division 4 (West North Central) Missouri, North Dakota, South Dakota, Nebraska, Kansas, Minnesota, Iowa
  • Region 3 (South)
    • Division 5 (South Atlantic) Delaware, Maryland, District of Columbia, Virginia, West Virginia, North Carolina, South Carolina, Georgia, Florida
    • Division 6 (East South Central) Kentucky, Tennessee, Mississippi, Alabama
    • Division 7 (West South Central) Oklahoma, Texas, Arkansas, Louisiana
  • Region 4 (West)
    • Division 8 (Mountain) Idaho, Montana, Wyoming, Nevada, Utah, Colorado, Arizona, New Mexico
    • Division 9 (Pacific) Alaska, Washington, Oregon, California, Hawaii
  
  

  U.S. Census Bureau. Census Bureau Regions and Divisions with State FIPS Codes. http://www2.census.gov/geo/pdfs/maps-data/maps/reference/us_regdiv.pdf. Accessed August 8, 2015.

• A metropolitan statistical area is considered urban, and a non-metro statistical area is rural.
• Teaching hospitals have an AMA-approved residency program, are a member of the Council of Teaching Hospitals (COTH) or have a ratio of full-time equivalent interns and residents to beds of .25 or higher.
• Control categories include government nonfederal (public), private not-for-profit (voluntary), and private investor-owned (proprietary). When there were enough hospitals of each type to allow it, hospitals were stratified as public, voluntary, and proprietary. This stratification was used for Southern rural, Southern urban nonteaching, and Western urban nonteaching. For smaller strata, the Midwestern rural and Western rural hospitals, a collapsed stratification of public versus private was used, with the voluntary and proprietary hospitals combined to form to form a single "private" category. For all other combinations of region, location, and teaching status, no stratification based on control was advisable given the number of hospitals in these cells.
• Bedsize assesses the number of short-term acute beds in a hospital.

The hospital's bedsize category is nested within census region, location, and teaching status.

Some strata were combined for sampling and weight calculations. Consequently, a given hospital's actual value for a stratifier may differ from those indicated by the value of NIS_STRATUM. Each hospital's individual stratifier values are contained in separate variables:

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