STRATUM - Stratum used to post-stratify hospital
Documentation Sections:
General Notes
Uniform Values
State Specific Notes
General Notes

STRATUM is a four-digit stratum identifier used to post-stratify hospitals for the calculation of universe and frame weights. Beginning in 1998, this data element is named NIS_STRATUM. Another stratum identifier (STRAT_ST) was used for the calculation of state weights. Strata had to be collapsed much more often for state weights than for universe and frame weights.

The following defines the 4 digit stratum number:

Digit Stratum Values
1st Region 1=Northeast
2nd Control 1=Government, nonfederal
    2=Private, not-for-profit
    3=Private, investor owned
3rd Location/Teaching 1=Rural
    2=Urban nonteaching
    3=Urban teaching
4th Bedsize 1=Small


The hospital's census region, control category, location, teaching status, and bedsize were obtained from the AHA Annual Survey of Hospitals.

  • A metropolitan statistical area is considered urban, and a non-metro statistical area is rural.
  • Teaching hospitals have an AMA-approved residency program or have membership in the Council of Teaching Hospitals.
  • Bedsize assesses the number of short-term acute beds in a hospital.

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

Location and Teaching Status Bedsize Small Medium Large
Rural 1-49 50-99 100+
Urban, nonteaching 1-99 100-199 200+
Urban, teaching 1-299 300-499 500+


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 STRATUM. Each hospital's actual values of stratifiers are contained in separate variables:

Stratifier Release 1 (1988-1992) Release 2 - Release 6 (1993-1997)
Ownership/Control ST_OWNER H_CONTRL
Location/Teaching LOCTEACH H_LOCTCH


Hospital stratum (STRATUM) is missing for zero-weight hospitals (see the File Composition for the Nationwide Inpatient Sample for a definition of zero-weight hospitals). Zero-weight hospitals are included in the 1988-1992 data for NIS, Release 1. Because relatively few hospitals were affected and the complexity of including these hospitals entailed considerable processing burden and costs, no zero-weight hospitals are included in the NIS after 1992.

Uniform Values
VariableDescriptionValueValue Description
STRATUMStratum used to post-stratify hospitalnnnn4 digit number - see gennote for more detail
State Specific Notes

Internet Citation: HCUP KID Description of Data Elements. Healthcare Cost and Utilization Project (HCUP). September 2008. Agency for Healthcare Research and Quality, Rockville, MD.
Are you having problems viewing or printing pages on this website?
If you have comments, suggestions, and/or questions, please contact
Privacy Notice, Viewers & Players
Last modified 9/17/08