NEDS_STRATUM is a five-digit stratum identifier used to sample hospitals for the NEDS. NEDS_STRATUM is based on geographic region, trauma designation, urban-rural location, teaching status, and hospital control. NEDS_STRATUM is also used to post-stratify hospitals for the calculation of discharge and hospital weights.

The following defines the five-digit stratum number:

• 1st digit - Region
• (1) Northeast
• (2) Midwest
• (3) South
• (4) West
• 2nd digit - Trauma center level
• Beginning in 2006:
• (0) not a trauma center
• (1) trauma center level I
• (2) trauma center level II
• (3) trauma center level III
• (8) trauma center level I or II, collapsed category
• (9) trauma center level I, II, or III, collapsed category
• 3rd digit - Urban-rural location
• (1) large metropolitan areas with at least 1 million residents
• (2) small metropolitan areas with less than 1 million residents
• (3) micropolitan areas
• (4) not metropolitan or micropolitan (non-urban residual)
• (6) collapsed category of any urban-rural location, beginning in the 2014 NEDS
• (7) collapsed category of small metropolitan and micropolitan, beginning in 2011 NEDS
• (8) metropolitan, collapsed category of large and small metropolitan
• (9) non-metropolitan, collapsed category of micropolitan and non-urban
• 4th digit - Teaching status
• (0) Metropolitan non-teaching
• (1) Metropolitan teaching
• (2) Non-metropolitan teaching and non-teaching
• 5th digit - Control
• (0) All, combines public, voluntary, and private
• (1) Public - government, nonfederal
• (2) Voluntary - private, non-profit
• (3) Proprietary - private, investor-owned/for-profit
• (4) Private, combines private voluntary and proprietary

The hospital's census region, control category, and teaching status were obtained from the AHA Annual Survey of Hospitals. Teaching hospitals have an AMA-approved residency program or have membership in the Council of Teaching Hospitals.

The urban-rural designation is based on the county of the hospital, as identified by the AHA. The four-level categorization is a simplified adaptation of the 2003 version of the Urban Influence Codes (UIC). The 12 categories of the UIC are combined into four broader categories that differentiate between large and small metropolitan, micropolitan, and a non-urban residual.

The trauma level designation is based on information from the Trauma Information Exchange Program database (TIEP), a national inventory of trauma centers in the U.S. Information is collected by the American Trauma Society and the Johns Hopkins Center for Injury Research and Policy and funded by the Centers for Disease Control and Prevention ^1,2. The TIEP database is updated quarterly and identifies all U.S. hospitals that are designated as trauma centers by a State or regional authority or verified by the American College of Surgeons' Committee on Trauma (ACS/COT). Designation of trauma center levels I, II, and III are based on criteria developed by the ACS/COT. Level I and II centers have comprehensive resources and are able to care for the most severely injured. Level I centers also provide leadership in education and research. Level III centers provide prompt assessment and resuscitation, emergency surgery and, if needed, transfer to a level I or II center. Level IV and V centers are State-defined and often located in remote areas. These centers resuscitate and stabilize patients and arrange transfer to an appropriate trauma facility. For the NEDS, facilities with levels I, II and III were identified as a trauma center. Level IV and V centers were set aside within the context of these data because many states choose not to designate hospitals at these levels of trauma care and their institutional characteristics have many similarities to community (non-trauma) hospitals in other areas. It is also important to note that while all level I, II, and III trauma centers offer a high level of trauma care, that there may be differences in the services and resources offered by hospitals of different levels. Further, hospitals of different levels may be utilized in diverse ways within the context of individual state trauma systems or the geographic areas in which they operate.

¹MacKenzie EJ, Hoyt DB, Sacra JC, et al. National inventory of hospital trauma centers. JAMA. 2003;289:1515-1522.
²American Trauma Society. Trauma Information Exchange Program. Available at: http://www.amtrauma.org/?page=TIEP. Accessed April 2005.