STRAT_ST is a four-digit stratum identifier used to post-stratify hospitals for the calculation of state weights. Another stratum identifier (STRATUM) was used for the calculation of universe and frame weights. Strata had to be collapsed much more often for state weights than for universe and frame weights. Beginning in the 1998 NIS, the data element STRAT_ST is not included in the data set.

The following defines the 4 digit stratum number:

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.

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

Hospital stratum (STRAT_ST) 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.