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, ownership/control, location/teaching, and bedsize. Information was 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, 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 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 actual values of stratifiers 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.