NRD_STRATUM is the stratum identifier used to post-stratify hospitals for the calculation of discharge weights. NRD_STRATUM includes the hospital's census region, urban/rural location, teaching status, size based on the number of beds, and ownership/control. Information on hospital characteristics was obtained from the AHA Annual Survey of Hospitals.

Strata were collapsed prior to the weight calculations to ensure that at least two sampled hospitals and at least 100 discharges were in each stratum. Strata were first collapsed across control/ownership, combining either the two private designations or all three types of control (public, private not-for-profit, and private for-profit). If the stratum combined across control still lacked a sufficient number of hospitals or discharges, then the location category was collapsed. Small and large metropolitan areas were combined or micropolitan and rural areas were combined. Lastly, if the stratum still lacked a sufficient number of hospitals or discharges, then the bed-size category was collapsed by combining the categories for large and medium hospitals. There was no collapsing of strata across region or teaching status.

The values of NRD_STRATUM differ from year to year. An individual stratum cannot be followed across data years.