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NIS Description of Data Elements
The NIS is set of longitudinal hospital inpatient databases included in the HCUP family. These databases are created by AHRQ through a Federal-State-Industry partnership.
 
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
    2=Midwest
    3=South
    4=West
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
    2=Medium
    3=Large

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)
Region ST_REG H_REGION
Ownership/Control ST_OWNER H_CONTRL
Location/Teaching LOCTEACH H_LOCTCH
Bedsize ST_BEDSZ H_BEDSZ

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.

 
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Uniform Values
VariableDescriptionValueValue Description
STRATUMStratum used to post-stratify hospitalnnnn4 digit number - see gennote for more detail
 
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State Specific Notes
None
 
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Internet Citation: HCUP NIS Description of Data Elements. Healthcare Cost and Utilization Project (HCUP). September 2008. Agency for Healthcare Research and Quality, Rockville, MD. www.hcup-us.ahrq.gov/db/vars/stratum/nisnote.jsp.
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Last modified 9/17/08