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:

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

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.