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Introduction to the HCUP Nationwide Inpatient Sample (NIS), 2003

HEALTHCARE COST AND UTLIZATION PROJECT – HCUP
A FEDERAL-STATE-INDUSTRY PARTNERSHIP IN HEALTH DATA

Sponsored by the Agency for Healthcare Research and Quality

 

 

INTRODUCTION TO

THE HCUP NATIONWIDE INPATIENT SAMPLE (NIS),

2003

 

 

These pages provide only an introduction to the NIS package.

  Full documentation is provided on the NIS Documentation CD-ROM.

For documentation updates and changes, be sure to visit: http://www.hcup-us.ahrq.gov.

 

Issued June 2005

Updated November 2015

 

Agency for Healthcare Research and Quality
Healthcare Cost and Utilization Project (HCUP

Phone: (866) 290-HCUP (4287)
E-mail: hcup@ahrq.gov
Website: http://www.hcup-us.ahrq.gov

 

NIS Data and Documentation Distributed by:
HCUP Central Distributor

Phone: (866) 556-4287 (toll-free)
Fax: (301) 628-3201
E-mail: hcup@s-3.com

Table of Contents

Index of Tables


HCUP NATIONWIDE INPATIENT SAMPLE ( NIS)
SUMMARY OF DATA USE LIMITATIONS

***** REMINDER *****


All users of the NIS must take the on–line HCUP Data Use Agreement (DUA) training course, and read and sign a Data Use Agreement.

Authorized users of HCUP data agree to the following restrictions: ‡

  • Will not use the data for any purpose other than research or aggregate statistical reporting.

  • Will not re–release any data to unauthorized users.

  • Will not redistribute HCUP data by posting on any Website or other publically-accessible online repository.

  • Will not identify or attempt to identify any individual, including by the use of vulnerability analysis or penetration testing. Methods that could be used to identify individuals directly or indirectly shall not be disclosed or published.

  • Will not publish information that could identify individual establishments (e.g., hospitals) ) and will not contact establishments.

  • Will not use the data concerning individual establishments for commercial or competitive purposes involving those establishments and will not use the data to determine rights, benefits, or privileges of individual establishments.

  • Will not use data elements from the proprietary severity adjustment software packages (3M APR–DRGs, HSS APS–DRGs, and Truven Health Analytics Disease Staging) for any commercial purpose or to disassemble, decompile, or otherwise reverse engineer the proprietary software.

  • Will acknowledge in reports that data from the "Healthcare Cost and Utilization Project (HCUP)", were used, including names of the specific databases used for analysis.

  • Will acknowledge that risk of individual identification of persons is increased when observations (i.e., individual discharge records) in any given cell of tabulated data is less than or equal to 10.

Any violation of the limitations in the Data Use Agreement is punishable under Federal law by a fine of up to $10,000 and up to 5 years in prison. Violations may also be subject to penalties under State statutes.

† The on–line Data Use Agreement training session and the Data Use Agreement are available on the HCUP User Support (HCUP–US) Website at http://www.hcup-us.ahrq.gov.
‡ Specific provisions are detailed in the Data Use Agreement for Nationwide Databases.


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HCUP CONTACT INFORMATION

All HCUP data users, including data purchasers and collaborators, must complete the online HCUP Data Use Agreement (DUA) Training Tool, and read and sign the HCUP Data Use Agreement. Proof of training completion and signed Data Use Agreements must be submitted to the HCUP Central Distributor as described below.

The on-line DUA training course is available at: http://www.hcup-us.ahrq.gov/tech_assist/dua.jsp.

The HCUP Nationwide Data Use Agreement are is available on the AHRQ-sponsored HCUP User Support (HCUP-US) Web site at:

http://www.hcup-us.ahrq.gov


HCUP Central Distributor

Data purchasers will be required to provide their DUA training completion code and will execute their DUAs electronically as a part of the online ordering process. The DUAs and training certificates for collaborators and others with access to HCUP data should be submitted directly to the HCUP Central Distributor using the contact information below.

The HCUP Central Distributor can also help with questions concerning HCUP database purchases, your current order, training certificate codes, or invoices, if your questions are not covered in the Purchasing FAQs on the HCUP Central Distributor Web site.

HCUP User Support:

Information about the content of the HCUP databases is available on the HCUP User Support (HCUP-US) Web site (http://www.hcup-us.ahrq.gov). If you have questions about using the HCUP databases, software tools, supplemental files, and other HCUP products, please review the HCUP Frequently Asked Questions or contact HCUP User Support:


WHAT’S NEW IN THE 2003
NATIONWIDE INPATIENT SAMPLE (NIS)?

  • Two States have joined the NIS in 2003: Indiana and New Hampshire. Data from Maine were not available for inclusion in the 2003 NIS.
  • The data element HOSPSTCO (the hospital’s county) is no longer present on the Core file but remains on the hospital file.
  • The data elements MDNUM1_R and MDNUM2_R (reidentified physician numbers) replace previous physician number data elements.
  • New data elements include the following:
    • ASOURCEUB92 indicates the source of admission and uses the same coding as the source of admission data element on the UB-92 claim form. ASOURCEUB92 has more detailed categories for routine admissions and transfers from other health facilities than the HCUP data element ASOURCE.
    • ECODEn is an array of the external cause of injury codes (i.e., “E codes”). Prior to 2003, E codes are included in the diagnosis array (DXn). Beginning in 2003, any separately reported E codes and any E codes encountered in the diagnosis array are placed in a separate array specific to E codes (ECODEn). NECODES indicates the total number of E codes (valid and invalid) that are coded on the original discharge record. The array E_CCSn contains the Clinical Classifications Software (CCS) category for the corresponding E code in the ECODEn array. The CCS consists of 21 categories of E codes based on a classification system developed by the Centers for Disease Control.
    • PT_UR_CAT4 is an urban-rural measure that characterizes the patient’s county of residence. The measure is a consolidated version of the Urban Influence Codes (UIC) developed by the U.S. Department of Agriculture, which expands upon the Metropolitan Statistical Area (MSA) classification as applied to the 2000 Census. In this classification, urban counties are distinguished by the population of their largest metropolitan area. Rural counties are differentiated by the size of the largest metropolitan area they are adjacent to and the size of their largest community.
    • ZIPINC_QRTL is a categorical variable indicating the quartile that contains the median household income of the patient’s ZIP Code of residence. For 2003, the median income quartiles are defined as: $1 - $35,999; $36,000 - $44,999; $45,000 - $59,999; and $60,000 or more.

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HEALTHCARE COST AND UTILIZATION PROJECT – HCUP
A FEDERAL-STATE-INDUSTRY PARTNERSHIP IN HEALTH DATA

Sponsored by the Agency for Healthcare Research and Quality

The Agency for Healthcare Research and Quality and
the staff of the Healthcare Cost and Utilization Project (HCUP)
thank you for purchasing the HCUP Nationwide Inpatient Sample ( NIS).



HCUP Nationwide Inpatient Sample ( NIS)

ABSTRACT

The Nationwide Inpatient Sample (NIS) is part of the Healthcare Cost and Utilization Project (HCUP), sponsored by the Agency for Healthcare Research and Quality (AHRQ), formerly the Agency for Health Care Policy and Research.

The NIS is a database of hospital inpatient stays. Researchers and policymakers use the NIS to identify, track, and analyze national trends in health care utilization, access, charges, quality, and outcomes.

The NIS is the largest all-payer inpatient care database that is publicly available in the United States, containing data from 5 to 8 million hospital stays from about 1,000 hospitals sampled to approximate a 20-percent stratified sample of U.S. community hospitals. The NIS is available for a 16-year time period, from 1988 to 2003, allowing analysis of trends over time. (Analyses of time trends are recommended from 1993 forward; see Using the HCUP Nationwide Inpatient Sample to Estimate Trends, a report available on the HCUP User Support Website and on the NIS Documentation CD-ROM.)

The NIS is the only national hospital database with charge information on all patients, regardless of payer, including persons covered by Medicare, Medicaid, private insurance, and the uninsured. The NIS's large sample size enables analyses of rare conditions, such as congenital anomalies; uncommon treatments, such as organ transplantation; and special patient populations, such as the uninsured.

Inpatient stay records in the NIS include clinical and resource use information typically available from discharge abstracts. Hospital and discharge weights are provided for producing national estimates. The NIS can be linked to hospital-level data from the American Hospital Association's Annual Survey of Hospitals and county-level data from the Bureau of Health Professions' Area Resource File, except in those states that do not allow the release of hospital identifiers.

Beginning in 1998, the NIS differs from previous NIS releases: some data elements were dropped, some were added, for some data elements, the coding was changed, and the sampling and weighting strategy was revised to improve the representativeness of the data. (See the report, Changes in the NIS Sampling and Weighting Strategy for 1998, which describes these changes, available on the HCUP User Support Website and on the NIS Documentation CD-ROM.)

Access to the NIS is open to users who sign data use agreements. Uses are limited to research and aggregate statistical reporting.

For more information on the NIS, visit the AHRQ-sponsored HCUP User Support (HCUP-US) Website at http://www.hcup-us.ahrq.gov or see the detailed documentation on the documentation CD that accompanies the data.

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INTRODUCTION TO THE HCUP NATIONWIDE INPATIENT SAMPLE ( NIS )

OVERVIEW OF NIS DATA

The Nationwide Inpatient Sample (NIS) contains all-payer data on hospital inpatient stays from States participating in the Healthcare Cost and Utilization Project (HCUP). Each year of the NIS provides information on approximately 5 million to 8 million inpatient stays from about 1,000 hospitals. All discharges from sampled hospitals are included in the NIS database.

The NIS contains patient-level clinical and resource use information included in a typical discharge abstract. The NIS can be linked directly to hospital-level data from the American Hospital Association (AHA) Annual Survey of Hospitals and to county-level data from the Health Resources and Services Administration Bureau of Health Professions’ Area Resource File (ARF), except in those states that do not allow the release of hospital identifiers.

The NIS is designed to approximate a 20-percent sample of U.S. community hospitals, defined by the AHA to be “all nonfederal, short-term, general, and other specialty hospitals, excluding hospital units of institutions.” Included among community hospitals are specialty hospitals such as obstetrics-gynecology, ear-nose-throat, short-term rehabilitation, orthopedic, and pediatric institutions. Also included are public hospitals and academic medical centers. Excluded are short-term rehabilitation hospitals (beginning with 1998 data), long-term hospitals, psychiatric hospitals, and alcoholism/chemical dependency treatment facilities.

This universe of U.S. community hospitals is divided into strata using five hospital characteristics: ownership/control, bed size, teaching status, urban/rural location, and U.S. region.

The NIS is a stratified probability sample of hospitals in the frame, with sampling probabilities proportional to the number of U.S. community hospitals in each stratum. The frame is limited by the availability of inpatient data from the data sources.

In order to improve the representativeness of the NIS, the sampling and weighting strategy was modified beginning with the 1998 data. The full description of this process can be found in the special report on Changes in NIS Sampling and Weighting Strategy for 1998.This report is available on the 2003 NIS Documentation CD-ROM and on the AHRQ-sponsored HCUP User Support (HCUP-US) Website at http://www.hcup-us.ahrq.gov/db/nation/nis/reports/Changes_in_NIS_Design_1998.pdf. To facilitate the production of national estimates, both hospital and discharge weights are provided, along with information necessary to calculate the variance of estimates. Detailed information on the design of the NIS is available in the year-specific special reports on Design of the Nationwide Inpatient Samplefound on the HCUP-US website and the NIS Documentation CD-ROM.

NIS data sets are currently available for multiple years, as shown in Table 1. Each release of the NIS includes:

Table 1. Summary of NIS Releases
 Data from   Media/format options   Structure of Releases
  • 1988-1992
  • 8 States in 1988
  • 11 States in 1989-1992
  On CD-ROM,
in ASCII format
5 years of data in a 6-CD set, compressed files
Two 10% subsamples of discharges for each year
  • 1993
  • 17 states
  • 1994
  • 17 states
  • 1995
  • 19 states
  • 1996
  • 19 states
  • 1997
  • 22 states
  • 1998
  • 22 states
  • 1999
  • 24 states
  • 2000
  • 28 states
  • 2001
  • 33 states
  • 2002
  • 35 states
  • 2003
  • 37 states
bracket spanning the years 1993-2003 for the media format: ON CD-Rom, in ASCII format On CD-ROM,
in ASCII format
1 year of data in a 2-CD set,
compressed files

Two 10% subsamples of discharges for each year

Beginning in 2002, a companion file with four different sets of severity measures

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NIS Data Sources, Hospitals, and Inpatient Stays

Table 2 summarizes the data sources, number of hospitals, and number of unweighted and weighted inpatient stays in NIS data.

Table 2. Summary of NIS Data Sources, Hospitals and Inpatient Stays, 1988-2003
Year Data sources Number of hospitals Number of discharges
in the NIS, unweighted
Number of discharges in the NIS, weighted for national estimates
1988 CA CO FL IL IA MA NJ WA 759 5,265,756 35,171,448
1989 AZ CA CO FL IL IA MA NJ PA WA WI
(Added AZ, PA, WI)
882 6,110,064 35,104,645
1990 AZ CA CO FL IL IA MA NJ PA WA WI
(No change)
871 6,268,515 35,215,397
1991 AZ CA CO FL IL IA MA NJ PA WA WI
(No change)
859 6,156,188 35,036,492
1992 AZ CA CO FL IL IA MA NJ PA WA WI
(No change)
856 6,195,744 35,011,385
1993 AZ CA CO CT FL IL IA KS MD MA
NJ NY OR PA SC WA WI
(Added CT, KS, MD, NY, OR, SC)
913 6,538,976 34,714,530
1994 AZ CA CO CT FL IL IA KS MD MA
NJ NY OR PA SC WA WI
(No change)
904 6,385,011 34,622,203
1995 AZ CA CO CT FL IL IA KS MD MA
MO NJ NY OR PA SC TN WA WI
(Added MO, TN)
938 6,714,935 34,791,998
1996 AZ CA CO CT FL IL IA KS MD MA
MO NJ NY OR PA SC TN WA WI
(No change)
906 6,542,069 34,874,386
1997 AZ CA CO CT FL GA HI IL IA KS MD MA
MO NJ NY OR PA SC TN UT WA WI
(Added GA, HI, UT)
1,012 7,148,420 35,408,207
1998 AZ CA CO CT FL GA HI IL IA KS MD MA
MO NJ NY OR PA SC TN UT WA WI
(No change)
984 6,827,350 34,874,001
1999 AZ CA CO CT FL GA HI IL IA KS MD MA ME MO NJ NY OR PA SC TN UT VA WA WI
(Added ME, VA)
984 7,198,929 35,467,673
2000 AZ CA CO CT FL GA HI IL IA KS KY MD
MA ME MO NC NJ NY OR PA SC TN TX
UT VA WA WI WV
(Added KY, NC, TX, WV)
994 7,450,992 36,417,565
2001 AZ CA CO CT FL GA HI IL IA KS KY MD MA ME MI MN MO NC NE NJ NY OR PA RI SC TN TX UT VA VT WA WI WV
(Added MI, MN, NE, RI, VT)
986 7,452,727
37,187,641
2002 CA CO CT FL GA HI IL IA KS KY MD MA ME MI MN MO NC NE NJ NY NV OH OR PA RI SC SD TN TX UT VA VT WA WI WV
(Added NV, OH, SD; AZ data were not available)
995 7,853,982
37,804,021
2003 AZ CA CO CT FL GA HI IL IN IA KS KY MD MA MI MN MO NC NE NH NJ NY NV OH OR PA RI SC SD TN TX UT VA VT WA WI WV
(Added AZ, IN, NH; ME data were not available)
994 7,977,728 38,220,659

State-Specific Restrictions

Some data sources that contributed data to the NIS imposed restrictions on the release of certain data elements or on the number and types of hospitals that could be included in the database. Because of confidentiality laws, some data sources were prohibited from providing HCUP with discharge records that indicated specific medical conditions, such as HIV/AIDS or behavioral health. Detailed information on these state-specific restrictions is available in the report on Sources of NIS Data and State-specific Restrictions on the NIS Documentation CD-ROM.

Contents of CD-ROM Set

There are two types of files included in the NIS: 1) data files and 2) documentation and tools files.

  1. Data Files - four types of fixed-width ASCII formatted data files are included in the NIS, beginning with the 2002 data:

    Inpatient Core File:This inpatient discharge-level file contains data for 100% of the discharges from a sample of hospitals in participating states. The unit of observation is an inpatient stay record. See Table 3 for a list of data elements in the Inpatient Core File. This file is available in all years.

    Subsample Inpatient Core Files:Each of these discharge-level files contains all data elements from the Core File for a 10% subsample of discharges from the NIS; these can be combined to create a 20% NIS subsample. These files can be useful for testing programs or validating models. The unit of observation is an inpatient stay record. See Table 3 for a list of data elements in the Inpatient Core Files. These files are available in all years.

    Hospital Weights File:This hospital-level file contains one observation for each hospital included in the NIS and contains weights and variance estimation data elements, as well as linkage data elements. The unit of observation is the hospital. The HCUP hospital identifier (HOSPID) provides the linkage between the NIS Inpatient Core files and the Hospital Weights file. See Table 4 for a list of data elements in the Hospital Weights File. This file is available in all years.

    Disease Severity Measures Files:These discharge-level files contain information from four different sets of disease severity measures. Information from these severity files is to be used in conjunction with the inpatient Core files. There is one severity measure file for each of the Core and subsample Core files. The unit of observation is an inpatient stay record. The HCUP unique record identifier (KEY) provides the linkage between the Core files and the Disease Severity Measures files. See Table 5 for a list of data elements in the Severity Measures Files. These files are available beginning with the 2002 NIS.
  2. Documentation and Tools Files

    Documentation:Complete file documentation, variable notes, and summary statistics are provided in a series of Portable Document Format (*.pdf) files. These files are detailed in Table 7.

    SAS source code:Code is included for the format library for the variables and for loading ASCII data into SAS format.

    SPSS source code:
    Code is included for the variable library and for loading ASCII data into SPSS format.

    Labels:Labels for the Clinical Classifications Software (CCS), formerly called the Clinical Classifications for Health Policy Research (CCHPR), and for the Diagnosis-Related Groups (multiple versions).

    File Specifications:Record layouts for all data files.

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NIS Data Elements

All releases of the NIS contain two types of data: inpatient stay records and hospital information with weights. Table 3 and Table 4 (on the following pages) identify the data elements that can be found in the inpatient stay and hospital weights files, respectively. Beginning with the 2002 NIS, an additional file including four different sets of disease severity measures is available. Table 5 identifies the data elements in the disease severity measures files.

Not all data elements in the NIS are uniformly coded or available across all States. This is not complete documentation for the data; please refer to the NIS Documentation CD-ROM for full documentation on all data elements, for summary statistics, and for the record layout.

Table 3. Data Elements in the NIS Inpatient Core Files

Data elements that are italicized are not included in the 2003 NIS Inpatient Core Files, but are only available in previous years’ files.

Type of
Data Element
HCUP
Variable Name
Years Available Coding Notes Unavailable in 2003 for:
Admission day of week or weekend AWEEKEND 1998-2003 Admission on weekend: (0) admission on Monday-Friday, (1) admission on Saturday-Sunday  
ADAYWK 1988-1997 Admission day of week: (1) Sunday, (2) Monday, (3) Tuesday, (4) Wednesday, etc.  
Admission month AMONTH 1988-2003 Admission month coded from (1) January to (12) December FL
Admission source ASOURCE 1988-2003 Admission source, uniform coding: (1) ER, (2) another hospital, (3) another facility including long-term care, (4) court/law enforcement, (5) routine/birth/other KS
ASOURCE_X 1998-2003 Admission source, as received from data source using State-specific coding KS
ASOURCEUB92 2003 Admission source (UB-92 standard coding). For newborn admissions (ATYPE = 4): (1) normal delivery, (2) premature delivery, (3) sick baby, (4) extramural birth; For non-newborn admissions (ATYPE NE 4): (1) physician referral, (2) clinic referral, (3) HMO referral, (4) transfer from a hospital, (5) transfer from a skilled nursing facility, (6) transfer from a another health care facility, (7) emergency room, (8) court/law enforcement, (A) transfer from a critical access hospital  
Admission type ATYPE 1988-2003 Admission type, uniform coding: (1) emergency, (2) urgent, (3) elective, (4) newborn, (5) trauma center beginning in 2003 data, (6) other CA
ELECTIVE 2002-2003 Indicates elective admission: (1) elective, (0) non-elective admission  
Age at admission AGE 1988-2003 Age in years coded 0-124 years  
AGEDAY 1988-2003 Age in days coded 0-365 only when the age in years is less than 1 FL, MA, NH, SC, TX
Clinical Classifications Software (CCS) category DXCCS1 - DXCCS15 1998-2003 CCS category for all diagnoses for NIS beginning in 1998  
DCCHPR1 1988-1997 CCS category for principal diagnosis for NIS prior to 1998. CCS was formerly called the Clinical Classifications for Health Policy Research (CCHPR)  
PRCCS1 - PRCCS15 1998-2003 CCS category for all procedures for NIS beginning in 1998  
PCCHPR1 1988-1997 CCS category for principal procedure for NIS prior to 1998. CCS was formerly called the Clinical Classifications for Health Policy Research (CCHPR)  
Data source information DSNUM 1988-1997 Data source number  
DSTYPE 1988-1997 Data source type: (1) State data organization, (2) Hospital association, (3) Consortia  
Diagnosis information DX1 - DX15 1988-2003 Diagnoses, principal and secondary (ICD-9-CM). Beginning in 2003, the diagnosis array does not include any of external cause of injury codes. These codes have been stored in a separate array ECODEn.  
NDX 1988-2003 Number of diagnoses coded on the original record  
DSNDX 1988-1997 Number of diagnosis fields provided by the data source  
DXSYS 1988-1997 Diagnosis system (ICD-9-CM)  
DXV1 - DXV15 1988-1997 Diagnosis validity flags  
Diagnosis Related Group (DRG) DRG 1988-2003 DRG in use on discharge date  
DRGVER 1988-2003 Grouper version in use on discharge date  
DRG10 1988-1999 DRG Version 10 (effective October 1992 - September 1993)  
DRG18 1998-2003 DRG Version 18 (effective October 2000 - September 2001)  
Discharge quarter DQTR 1988-2003 Coded: (1) Jan - Mar, (2) Apr - Jun, (3) Jul - Sep, (4) Oct - Dec  
Discharge weights
(Weights for 1988-1993 are on Hospital Weights file)
DISCWT 1998-2003 Discharge weight on Core file and Hospital Weights file for NIS beginning in 1998. In all data years except 2000, this weight is used to create national estimates for all analyses. In 2000 only, this weight is used to create national estimates for all analyses excluding those that involve total charges.  
DISCWT_U 1993-1997 Discharge weight on Core file and Hospital Weights file for NIS prior to 1998  
DISCWTcharge 2000 Discharge weight for national estimates of total charges. In 2000 only, this weight is used to create national estimates for analyses that involve total charges  
DISCWT10 1998-2003 Discharge weight on 10% subsample Core file for NIS beginning in 1998. In all data years except 2000, this weight is used to create national estimates for all analyses. In 2000 only, this weight is used to create national estimates for all analyses excluding those that involve total charges  
D10CWT_U 1993-1997 Discharge weight on 10% subsample Core file for NIS prior to 1998  
DISCWTcharge10 2000 Discharge weight for national estimates of total charges on 10% subsample file. In 2000 only, this weight is used to create national estimates for analyses that involve total charges  
Discharge year YEAR 1988-2003    
Disposition of patient (discharge status) DISP 1988-1997 Disposition of patient, uniform coding used prior to 1998: (1) routine, (2) short-term hospital, (3) skilled nursing facility, (4) intermediate care facility, (5) another type of facility, (6) home health care, (7) against medical advice, (20) died  
DIED 1988-2003 Indicates in-hospital death: (0) did not die during hospitalization, (1) died during hospitalization  
DISPUB92 1998-2003 Disposition of patient, UB-92 coding: (1) routine, (2) short term hospital, (3) skilled nursing facility, (4) intermediate care, (5) another type of facility, (6) home health care, (7) against medical advice, (8) home IV provider, (20) died in hospital, (40) died at home, (41) died in a medical facility, (42) died, place unknown, (43) Federal health facility beginning in 2003 data, (50) Hospice, home, (51) Hospice, medical facility, (61) hospital-based Medicare approved swing bed beginning in 2000 data, (62) another rehabilitation facility beginning in 2001 data, (63) long term care hospital beginning in 2001 data, (64) certified nursing facility beginning in 2002 data, (71) another institution for outpatient services beginning in 2000 data, (72) this institution for outpatient services beginning in 2000 data, (99) discharged alive, destination unknown beginning in 2001 data CA, MD
DISPUNIFORM 1998-2003 Disposition of patient, uniform coding used beginning in 1998: (1) routine, (2) transfer to short term hospital, (5) other transfers, including skilled nursing facility, intermediate care, and another type of facility, (6) home health care, (7) against medical advice, (20) died in hospital, (99) discharged alive, destination unknown  
External causes of injury and poisoning ECODE1 - ECODE4 2003 External cause of injury and poisoning code, primary and secondary (ICD-9-CM). Beginning in 2003, external cause of injury codes are stored in a separate array ECODEn from the diagnosis codes in the array DXn. Prior to 2003, these codes are contained in the diagnosis array (DXn).  
E_CCS1 - E_CCS4 2003 CCS category for the external cause of injury and poisoning codes  
NECODE 2003 Number of external cause of injury codes on the original record. A maximum of 4 codes are retained on the NIS.  
Gender of patient FEMALE 1998-2003 Indicates gender for NIS beginning in 1998: (0) male, (1) female  
SEX 1988-1997 Indicates gender for NIS prior to 1998: (1) male, (2) female  
Hospital information DSHOSPID 1988-2003 Hospital number as received from the data source GA, HI, IN, KS, MI, NE, OH, SC, SD, TN, TX
HOSPID 1988-2003 HCUP hospital number (links to Hospital Weights file)  
HOSPST 1988-2003 State postal code for the hospital (e.g., AZ for Arizona)  
HOSPSTCO 1988-2002 Modified Federal Information Processing Standards (FIPS) State/county code for the hospital links to Area Resource File (available from the Bureau of Health Professions, Health Resources and Services Administration) Beginning in 2003, this data element is available only on the hospital file.  
NIS_STRATUM 1998-2003 Stratum used to sample hospitals, based on geographic region, control, location/teaching status, and bed size. Stratum information is also in the Hospital Weights file.  
Length of Stay LOS 1988-2003 Length of stay, edited  
LOS_X 1988-2003 Length of stay, as received from data source  
Location of the patient PL_UR_CAT4 2003 Urban–rural designation for patient’s county of residence: (1) large metropolitan, (2) small metropolitan, (3) micropolitan, (4) non-core  
Major Diagnosis Category (MDC) MDC 1988-2003 MDC in use on discharge date  
MDC10 1988-1999 MDC Version 10 (effective October 1992 - September 1993)  
MDC18 1998-2003 MDC Version 18 (effective October 2000 - September 2001)  
Median household income for patient's ZIP Code ZIPINC_QRTL 2003 Median household income quartiles for patient's ZIP Code. For 2003, the median income quartiles are defined as: $1 - $35,999; $36,000 - $44,999; $45,000 - $59,999; and $60,000 or more.  
ZIPINC 1998-2002 Median household income category in files beginning in 1998: (1) $1-$24,999, (2) $25,000-$34,999, (3) $35,000-$44,999, (4) $45,000 and above  
ZIPINC4 1988-1997 Median household income category in files prior to 1998: (1) $1-$25,000, (2) $25,001-$30,000, (3) $30,001-$35,000, (4) $35,001 and above  
ZIPINC8 1988-1997 Median household income category in files prior to 1998: (1) $1-$15,000, (2) $15,001-$20,000, (3) $20,001-$25,000, (4) $25,001-$30,000, (5) $30,001-$35,000, (6) $35,001-$40,000, (7) $40,001-$45,000, (8) $45,001 or more  
Neonatal/ maternal flag NEOMAT 1988-2003 Assigned from diagnoses and procedure codes: (0) not maternal or neonatal, (1) maternal diagnosis or procedure, (2) neonatal diagnosis, (3) maternal and neonatal on same record  
Payer information PAY1 1988-2003 Expected primary payer, uniform: (1) Medicare, (2) Medicaid, (3) private including HMO, (4) self-pay, (5) no charge, (6) other  
PAY1_N 1988-1997 Expected primary payer, nonuniform: (1) Medicare, (2) Medicaid, (3) Blue Cross, Blue Cross PPO, (4) commercial, PPO, (5) HMO, PHP, etc., (6) self-pay, (7) no charge, (8) Title V, (9) Worker's Compensation, (10) CHAMPUS, CHAMPVA, (11) other government, (12) other  
PAY1_X 1998-2003 Expected primary payer, as received from the data source  
PAY2 1988-2003 Expected secondary payer, uniform: (1) Medicare, (2) Medicaid, (3) private including HMO, (4) self-pay, (5) no charge, (6) other CA, CO, FL, GA, HI, IA, OH, RI, SD, VA
PAY2_N 1988-1997 Expected secondary payer, nonuniform: (1) Medicare, (2) Medicaid, (3) Blue Cross, Blue Cross PPO, (4) commercial, PPO, (5) HMO, PHP, etc., (6) self-pay, (7) no charge, (8) Title V, (9) Worker's Compensation, (10) CHAMPUS, CHAMPVA, (11) other government, (12) other  
PAY2_X 1998-2003 Expected secondary payer, as received from the data source  
Physician identifiers,
synthetic
MDID_S 1988-2000 Synthetic attending physician number in files prior to 2001  
MDNUM1_R 2003 Re-identified attending physician number in files starting in 2003 CA, CT, GA, HI, IN, IL, KS, MA, NC, OH, UT, VT, WI
MDNUM1_S 2001-2002 Synthetic attending physician number in files beginning in 2001 and discontinued in 2003  
SURGID_S 1988-2000 Synthetic secondary physician number in files prior to 2001  
MDNUM2_R 2003 Re-identified secondary physician number in files starting in 2003 CA, CT, HI, IL, IN, KS, OH, WI
MDNUM2_S 2001-2002 Synthetic secondary physician number in files beginning in 2001 and discontinued in 2003  
Procedure information PR1 - PR15 1988-2003 Procedures, principal and secondary (ICD-9-CM)  
NPR 1988-2003 Number of procedures coded on the original record  
DSNPR 1988-1997 Number of procedure fields in this data source  
PRSYS 1988-1997 Procedure system (ICD-9-CM)  
PRV1 -PRV15 1988-1997 Procedure validity flag  
PRDAY1 1988-2003 Number of days from admission to principal procedure. IL, KS, OH, UT, WA, WV
PRDAY2 - PRDAY15 1998-2003 Number of days from admission to secondary procedures. IL, KS, OH, UT, WA, WV
Race of Patient RACE 1988-2003 Race, uniform coding: (1) white, (2) black, (3) Hispanic, (4) Asian or Pacific Islander, (5) Native American, (6) other GA, IL, KY, ME, MN, NE, NV, OH, OR, WA, WV
Record identifier, synthetic KEY 1998-2003 Unique record number for file beginning in 1998  
SEQ 1988-1997 Unique record number for NIS prior to 1998  
SEQ_SID 1988-1997 Unique record number for NIS prior to 1998  
PROCESS 1988-1997 Processing number for NIS prior to 1998  
Total Charges TOTCHG 1988-2003 Total charges, edited  
TOTCHG_X 1988-2003 Total charges, as received from data source  

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Table 4. Data Elements in the NIS Hospital Weights File

Data elements that are italicized are not included in the 2003 NIS Hospital Weights File, but are only available in previous years’ files.

Type of
Data Element
HCUP
Variable Name
Years Available Coding Notes Unavailable in 2003 for:
Discharge counts N_DISC_U 1988-1997 Number of AHA universe discharges in the stratum  
S_DISC_U 1988-2003 Number of sampled discharges in the sampling stratum (NIS_STRATUM or STRATUM)  
S_DISC_S 1988-1997 Number of sampled discharges in the stratum STRAT_ST  
N_DISC_F 1988-1997 Number of frame discharges in the stratum  
N_DISC_S 1988-1997 Number of State's discharges in the stratum  
TOTAL_DISC 1998-2003 Total number of discharges from this hospital in the NIS  
TOTDSCHG 1988-1997 Total number of discharges from this hospital in the NIS  
Discharge weights DISCWT 1998-2003 Discharge weight used in the NIS beginning in 1998. In all data years except 2000, this weight is used to create national estimates for all analyses. In 2000 only, this weight is used to create national estimates for all analyses excluding those that involve total charges.  
DISCWT_U 1988-1997 Discharge weights used in the NIS prior to 1998.  
DISCWT_F 1988-1997 Discharge weights to the sample frame are available only in 1988-1997  
DISCWT_S 1988-1997 Discharge weights to the State are available only in 1988-1997  
DISCWTcharge 2000 Discharge weight for national estimates of total charges for 2000 only.  
Discharge Year YEAR 1988-2003 Discharge year  
Hospital counts N_HOSP_F 1988-1997 Number of frame hospitals in the stratum  
N_HOSP_S 1988-1997 Number of State's hospitals in the stratum  
N_HOSP_U 1988-2003 Number of AHA universe hospitals in the stratum  
S_HOSP_S 1988-1997 Number of sampled hospitals in STRAT_ST  
S_HOSP_U 1988-2003 Number of sampled hospitals in the stratum (NIS_STRATUM or STRATUM)  
Hospital identifiers HOSPID 1988-2003 HCUP hospital number (links to inpatient Core
files)
 
AHAID 1988-2003 AHA hospital identifier that matches AHA Annual Survey of Hospitals (not available for all states) GA, HI, IN, KS, MI, NE, OH, SC, SD, TN, TX
IDNUMBER 1988-2003 AHA hospital identifier without the leading 6 (not available for all states) GA, HI, IN, KS, MI, NE, OH, SC, SD, TN, TX
HOSPNAME 1993-2003 Hospital name from AHA Annual Survey of Hospitals (not available for all states) GA, HI, IN, KS, MI, NE, OH, SC, SD, TN, TX
Hospital location HOSPADDR 1993-2003 Hospital address from AHA Annual Survey of Hospitals (not available for all states) GA, HI, IN, KS, MI, NE, OH, SC, SD, TN, TX
HOSPCITY 1993-2003 Hospital city from AHA Annual Survey of Hospitals (not available for all states) GA, HI, IN, KS, MI, NE, OH, SC, SD, TN, TX
HOSPST 1988-2003 Hospital state postal code for hospital (e.g., AZ for Arizona)  
HOSPSTCO 2002-2003 Modified Federal Information Processing Standards (FIPS) State/county code for the hospital links to Area Resource File (available from the Bureau of Health Professions, Health Resources and Services Administration) GA, HI, IN, KS, MI, NE, OH, SC, SD, TN, TX
HOSPZIP 1993-2003 Hospital ZIP Code from AHA Annual Survey of Hospitals (not available for all states) GA, HI, IN, KS, MI, NE, OH, SC, SD, TN, TX
Hospital characteristics HOSP_BEDSIZE 1998-2003 Bed size of hospital: (1) small, (2) medium, (3) large  
H_BEDSZ 1993-1997 Bed size of hospital: (1) small, (2) medium, (3) large  
ST_BEDSZ 1988-1992 Bed size of hospital: (1) small, (2) medium, (3) large  
HOSP_CONTROL 1998-2003 Control/ownership of hospital: (0) government or private, collapsed category, (1) government, nonfederal, public, (2) private, non-profit, voluntary, (3) private, invest-own, (4) private, collapsed category  
H_CONTRL 1993-1997 Control/ownership of hospital: (1) government, nonfederal (2) private, non-profit (3) private, invest-own  
ST_OWNER 1988-1992 Control/ownership of hospital: (1) public (2) private, non-profit (3) private for profit  
HOSP_LOCATION 1998-2003 Location: (0) rural, (1) urban  
H_LOC 1993-1997 Location: (0) rural, (1) urban  
HOSP_LOCTEACH 1998-2003 Location/teaching status of hospital: (1) rural, (2) urban non-teaching, (3) urban teaching  
H_LOCTCH 1993-1997 Location/teaching status of hospital: (1) rural, (2) urban non-teaching, (3) urban teaching  
LOCTEACH 1988-1992 Location/teaching status of hospital: (1) rural, (2) urban non-teaching, (3) urban teaching  
HOSP_REGION 1998-2003 Region of hospital: (1) Northeast, (2) Midwest, (3) South, (4) West  
H_REGION 1993-1997 Region of hospital: (1) Northeast, (2) Midwest, (3) South, (4) West  
ST_REG 1988-1992 Region of hospital: (1) Northeast, (2) Midwest, (3) South, (4) West  
HOSP_TEACH 1998-2003 Teaching status of hospital: (0) non-teaching, (1) teaching  
H_TCH 1993-1997 Teaching status of hospital: (0) non-teaching, (1) teaching  
NIS_STRATUM 1998-2003 Stratum used to sample hospitals beginning in 1998; includes geographic region, control, location/teaching status, and bed size  
STRATUM 1988-1997 Stratum used to sample hospitals prior to 1998; includes geographic region, control, location/teaching status, and bed size  
STRAT_ST 1988-1997 Stratum for State-specific weights  
Hospital weights HOSPWT 1998-2003 Weight to hospitals in AHA universe (i.e., total U.S.) beginning in 1998  
HOSPWT_U 1988-1997 Weight to hospitals in AHA universe (i.e., total U.S.) prior to 1998  
HOSPWT_F 1988-1997 Weight to hospitals in the sample frame.  
HOSPWT_S 1988-1997 Weight to hospitals in the State  

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Table 5. Data Elements in the NIS Disease Severity Measures Files

All data elements listed below are available in the 2003 NIS Disease Severity Measures Files.

Type of
Data Element
HCUP
Variable Name
Years Available Coding Notes Unavailable in 2003 for:
AHRQ Comorbidity Software (AHRQ) CM_AIDS 2002-2003 AHRQ comorbidity measure: Acquired immune deficiency syndrome PA
CM_ALCOHOL 2002-2003 AHRQ comorbidity measure: Alcohol abuse PA
CM_ANEMDEF 2002-2003 AHRQ comorbidity measure: Deficiency anemias PA
CM_ARTH 2002-2003 AHRQ comorbidity measure: Rheumatoid arthritis/collagen vascular diseases PA
CM_BLDLOSS 2002-2003 AHRQ comorbidity measure: Chronic blood loss anemia PA
CM_CHF 2002-2003 AHRQ comorbidity measure: Congestive heart failure PA
CM_CHRNLUNG 2002-2003 AHRQ comorbidity measure: Chronic pulmonary disease PA
CM_COAG 2002-2003 AHRQ comorbidity measure: Coagulopathy PA
CM_DEPRESS 2002-2003 AHRQ comorbidity measure: Depression PA
CM_DM 2002-2003 AHRQ comorbidity measure: Diabetes, uncomplicated PA
CM_DMCX 2002-2003 AHRQ comorbidity measure: Diabetes with chronic complications PA
CM_DRUG 2002-2003 AHRQ comorbidity measure: Drug abuse PA
CM_HTN_C 2002-2003 AHRQ comorbidity measure: Hypertension, uncomplicated and complicated PA
CM_HYPOTHY 2002-2003 AHRQ comorbidity measure: Hypothyroidism PA
CM_LIVER 2002-2003 AHRQ comorbidity measure: Liver disease PA
CM_LYMPH 2002-2003 AHRQ comorbidity measure: Lymphoma PA
CM_LYTES 2002-2003 AHRQ comorbidity measure: Fluid and electrolyte disorders PA
CM_METS 2002-2003 AHRQ comorbidity measure: Metastatic cancer PA
CM_NEURO 2002-2003 AHRQ comorbidity measure: Other neurological disorders PA
CM_OBESE 2002-2003 AHRQ comorbidity measure: Obesity PA
CM_PARA 2002-2003 AHRQ comorbidity measure: Paralysis PA
CM_PERIVASC 2002-2003 AHRQ comorbidity measure: Peripheral vascular disorders PA
CM_PSYCH 2002-2003 AHRQ comorbidity measure: Psychoses PA
CM_PULMCIRC 2002-2003 AHRQ comorbidity measure: Pulmonary circulation disorders PA
CM_RENLFAIL 2002-2003 AHRQ comorbidity measure: Renal failure PA
CM_TUMOR 2002-2003 AHRQ comorbidity measure: Solid tumor without metastasis PA
CM_ULCER 2002-2003 AHRQ comorbidity measure: Peptic ulcer disease excluding bleeding PA
CM_VALVE 2002-2003 AHRQ comorbidity measure: Valvular disease PA
CM_WGHTLOSS 2002-2003 AHRQ comorbidity measure: Weight loss PA
All Patient Refined DRG (3M) APRDRG 2002-2003 All Patient Refined DRG PA
APRDRG_Risk_Mortality 2002-2003 All Patient Refined DRG: Risk of Mortality Subclass PA
APRDRG_Severity 2002-2003 All Patient Refined DRG: Severity of Illness Subclass PA
All-Payer Severity-adjusted DRG (HSS, Inc.) APSDRG 2002-2003 All-Payer Severity-adjusted DRG PA
APSDRG_Mortality_Weight 2002-2003 All-Payer Severity-adjusted DRG: Mortality Weight PA
APSDRG_LOS_Weight 2002-2003 All-Payer Severity-adjusted DRG: Length of Stay Weight PA
APSDRG_Charge_Weight 2002-2003 All-Payer Severity-adjusted DRG: Charge Weight PA
Disease Staging (Medstat) DS_DX_Category1 2002-2003 Disease Staging: Principal Disease Category PA
DS_Stage1 2002-2003 Disease Staging: Stage of Principal Disease Category PA
DS_LOS_Level 2002-2003 Disease Staging: Length of Stay Level PA
DS_LOS_Scale 2002-2003 Disease Staging: Length of Stay Scale PA
DS_Mrt_Level 2002-2003 Disease Staging: Mortality Level PA
DS_Mrt_Scale 2002-2003 Disease Staging: Mortality Scale PA
DS_RD_Level 2002-2003 Disease Staging: Resource Demand Level PA
DS_RD_Scale 2002-2003 Disease Staging: Resource Demand Scale PA
Linkage Variables HOSPID 2002-2003 HCUP hospital identification number PA
KEY 2002-2003 HCUP record identifier PA

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SAMPLING OF HOSPITALS INCLUDED IN THE NIS

The hospital universe is defined by all hospitals that were open during any part of each calendar year and were designated as community hospitals in the AHA Annual Survey of Hospitals.

For more information on how hospitals in the data were mapped to hospitals as defined by the AHA, refer to the special report: HCUP Hospital Identifiers. For a list of all data sources, refer to: Sources of NIS Data and State-Specific Restrictions. For more detailed descriptions of the sampling design, refer to the year-specific special reports Design of the HCUP Nationwide Inpatient Sample. All reports can be found on the NIS Documentation CD-ROM.

Stratification Variables

To help ensure generalizability, five hospital sampling strata were defined based on hospital characteristics contained in the AHA Annual Survey of Hospitals. The stratification variables are:

Table 6. Bed Size Categories, by Region
Location and Teaching Status Hospital Bed size
Small Medium Large
NORTHEAST
Rural 1-49 50-99 100+
Urban,non-teaching 1-124 125-199 200+
Urban, teaching 1-249 250-424 425+
MIDWEST
Rural 1-29 30-49 50+
Urban, non-teaching 1-74 75-174 175+
Urban, teaching 1-249 250-374 375+
SOUTH
Rural 1-39 40-74 75+
Urban, non-teaching 1-99 100-199 200+
Urban, teaching 1-249 250-449 450+
WEST
Rural 1-24 25-44 45+
Urban, non-teaching 1-99 100-174 175+
Urban, teaching 1-199 200-324 325+

To further ensure geographic representativeness, implicit stratification variables included state and three-digit zip code (the first three digits of the hospital’s five digit zip code). The hospitals were sorted according to these variables prior to systematic random sampling. For more detailed descriptions of the stratification and sample design, refer to the year-specific special reports Design of the HCUP Nationwide Inpatient Sample which can be found on the HCUP-US Website and on the NIS Documentation CD-ROM.

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GETTING STARTED

NIS information is provided on two CD-ROMs. The NIS inpatient core and hospital weights data files are on CD-ROM #1, and the NIS documentation and tools and the disease severity measures files are on CD-ROM #2.

NIS Data Files

In order to load and analyze the NIS data onto your PC, you will need 10 gigabytes of space available. Because of the size of the files, the data are distributed as self-extracting PKZIP compressed files. To decompress the data, you should follow these steps:

  1. Create a directory for the NIS on your hard drive.
  2. Copy the self-extracting data files from the NIS Data Files CD-ROM into the new directory.
  3. Unzip each file by running the corresponding *.exe file.
    • Type the file name within DOS or click on the name within Windows Explorer.
    • Edit the name of the "Unzip To Folder" in the WinZip Self-Extractor dialog to select the desired destination directory for the extracted file.
    • Click on the "Unzip" button.

The ASCII data files will then be uncompressed into this directory. After the files are uncompressed, the *.exe files can be deleted.

NIS Documentation

NIS documentation files on the Documentation CD-ROM provide important resources for the user. Refer to these resources to understand the structure and content of the NIS and to aid in using the NIS. Many of the documentation files are provided in portable document format (*.pdf) files. Files with the *.pdf extension can be viewed, searched, and printed using the Adobe Acrobat Reader®.

You must have the Adobe Acrobat Reader software on your computer to access the NIS documentation. If you do not have Adobe Acrobat Reader software on your computer, see the DOCUMENTATION.README.TXT file on NIS Documentation CD-ROM for instructions on installing or obtaining the software.

The Acrobat Reader provided on the NIS Documentation CD-ROM is for IBM-compatible microcomputers running Microsoft Windows 98, second edition, or higher. More information and Acrobat Reader software for other platforms (DOS, Windows 3.1, Macintosh, Sun Systems, etc.) may be obtained free of charge from the Adobe Home Page at http://www.adobe.com/. For further assistance in installing and running the Adobe Acrobat Reader on your computer platform, please consult your local support personnel.

Table 7 describes the documentation, tools, and the severity adjustment data files that can be found on the NIS Documentation CD-ROM and illustrates the structure of the directories and subdirectories on the CD. All NIS documentation is also available on the AHRQ-sponsored HCUP User Support Website at http://www.hcup-us.ahrq.gov.

Table 7 . NIS Documentation CD-ROM, including Severity Adjustment Files
Directory Description
Root Includes DOCUMENTATION.README.TXT file with introductory information on accessing the NIS documentation
/Adobe Acrobat Reader Adobe Acrobat Reader files for IBM compatible for Microsoft Windows 95 or higher. (One text, one HTML, and one application file)
/Description of Data Elements Includes information on all NIS variables such as uniform coding and state-specific information: Core (for data elements starting with letters A-L and letters M-Z), Disease Severity Measures, and Hospital Weights. (PDF files)
/File Specifications Includes data set name, number of records, record length, and record layout. One file per data file: Core, Core Subsample #1, Core Subsample #2, Disease Severity Measures, Subsample #1 Disease Severity Measures, Subsample #2 Disease Severity Measures, and Hospital Weights. (Text files)
/General Information Includes:
  • Introduction to the NIS, 2003 (PDF file) – this document
  • Availability of NIS data elements across all years (PDF file)
  • Sources of NIS Data and State-Specific Restrictions (PDF file)
/HCUP Tools_Labels Includes:
  • Label file for the Clinical Classifications Software (CCS), a categorization scheme that groups ICD-9-CM diagnosis and procedure codes into mutually exclusive categories. (Text file)
  • Label file for Diagnosis Related Groups (DRGs), multiple versions provided (Text file)
  • SAS code to create format library of variable labels (Text file)
/SAS Load Programs SAS programming code to convert ASCII data files into SAS. One file per data file: Core, Core Subsample #1, Core Subsample #2, Disease Severity Measures, Subsample #1 Disease Severity Measures, Subsample #2 Disease Severity Measures, and Hospital Weights. (Text files)
/Severity Adjustment Files Includes self-extracting compressed data files with the four sets of severity measures: Disease Severity Measures, Subsample #1 Disease Severity Measures, Subsample #2 Disease Severity Measures.
/Severity Adjustment Reports Includes information on each of the severity measures: Overview of Severity Systems, documentation for APR-DRGS, documentation of APS-DRGS, and documentation for Disease Staging.
/Special Reports Includes:
  • Design of the Nationwide Inpatient Sample, 2003 (PDF file)
  • Changes in NIS Sampling and Weighting Strategy for 1998 (PDF file)
  • NIS Comparison Report (PDF file)
  • Calculating Nationwide Inpatient Sample Variances (PDF file)
  • Using the HCUP Nationwide Inpatient Sample to Estimate Trends
  • HCUP Coding Practices (PDF file)
  • HCUP Quality Control Procedures (PDF file)
  • HCUP Hospital Identifiers (PDF file)
/SPSS Load Programs SPSS programming code to convert ASCII data files into SPSS. One file per data file: Core, Core Subsample #1, Core Subsample #2, Disease Severity Measures, Subsample #1 Disease Severity Measures, Subsample #2 Disease Severity Measures, and Hospital Weights. (Text files)
/Summary Statistics Includes summary statistics (means and frequencies) on NIS data. At least one file per data file: Core (weighted and unweighted statistics and data quality report), Core Subsample #1, Core Subsample #2, Disease Severity Measures, Subsample #1 Disease Severity Measures, Subsample #2 Disease Severity Measures, and Hospital Weights. (PDF files)

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HOW TO USE THE NIS

This section provides a brief synopsis of special considerations when using the NIS. For more details see detailed documentation under Special Reports, Description of Data Elements, and Summary Statistics.

Creating National Estimates

NIS Year Name of Discharge Weight on the Core File to Use for Creating Nationwide Estimates Name of Discharge Weight on the 10% Subsample Core File to Use for Creating Nationwide Estimates
2001 - 2003
  • DISCWT for all analyses
  • DISCWT10 for all analyses
2000
  • DISCWT to create nationwide estimates for all analyses except those that involve total charges.
  • DISCWTCHARGE to create nationwide estimates of total charges. 
  • DISCWT10 to create nationwide estimates for all analyses except those that involve total charges.
  • DISCWTCHARGE10 to create nationwide estimates of total charges. 
1998 - 1999
  • DISCWT for all analyses
  • DISCWT10 for all analyses
1988 - 1997
  • DISCWT_U for all analyses
  • D10CWT_U for all analyses

Why the NIS Should not be Used to Make State-Level Estimates

AHRQ strongly advises researchers against using the NIS to estimate State-specific statistics. Prior to 2012, State is available as a NIS data element. However, these NIS samples were not designed to yield a representative sample of hospitals at the State level. AHRQ recommends that researchers employ the SID for State-level estimates.

Each NIS sample is drawn from the sampling frame consisting of discharge data submitted by HCUP Partners-statewide data organizations that agree to participate in the NIS. Data from non-Partner States are missing completely from the sampling frame, and data from Partner States are sometimes incomplete because of different State reporting requirements, different State restrictions, or other data omissions. The NIS is designed to represent hospitals and discharges nationally, including those outside the sampling frame.

To accomplish this, within each hospital sampling stratum the NIS draws a number of hospitals from the sampling frame required to net a total of 20 percent of hospitals nationally. The sampling strata are defined by census region (4 regions), hospital ownership (3 categories), urban-rural location, teaching status, and bed size (3 categories). As a result, the proportion of NIS hospitals in a stratum that are from a given State is unlikely to equal the State's actual proportion of hospitals in that stratum. Consequently, the sample of NIS hospitals is unlikely to be representative of hospitals in the State, and the NIS sample weights will not be appropriate at the State level.

The level of this "misrepresentation" varies across the States in any given year of the NIS, which further confounds State-to-State comparisons on the basis of State-specific estimates from the NIS. Moreover, for a given State the level of misrepresentation changes from year to year as States (and hospitals) enter and exit the sampling frame over time. This further confounds State-specific trends on the basis of State-specific estimates from the NIS.

Finally, because the NIS was not designed to be representative at the State level, design-based estimates of standard errors are not possible, which severely hampers State-level inferences. Moreover, the NIS is composed of all discharges from a sample of hospitals (a cluster sample). The hospital-to-hospital variation and the small number of hospitals available in the NIS for many States make Statelevel estimates very imprecise at best and biased at worst.

Studying Trends

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Choosing Data Elements for Analysis

ICD-9-CM Diagnosis and Procedure Codes

OTHER HCUP PRODUCTS

Information on HCUP products and services is available on the World Wide Web on the AHRQ Website https://www.ahrq.gov/research/data/hcup/index.html. HCUP User Support is available at http://www.hcup-us.ahrq.gov.

DATABASES

Nationwide Inpatient Sample (NIS)is a nationwide database of hospital inpatient stays. The NIS is the largest all-payer inpatient care database that is publicly available in the United States, containing data from 5 to 8 million hospital stays from about 1,000 hospitals sampled to approximate a 20-percent stratified sample of U.S. community hospitals. The NIS is available for a 16-year time period, from 1988 to 2003, allowing analysis of trends over time. For more information, visit the HCUP User Support Website at http://www.hcup-us.ahrq.gov or contact the HCUP Central Distributor (detailed below).

State Inpatient Databases (SID)are hospital inpatient databases from Data Organizations participating in HCUP.  The SID contain the universe of the inpatient discharge abstracts in the participating HCUP States, translated into a uniform format to facilitate multi-State comparisons and analyses. Together, the SID encompass about 90 percent of all U.S. community hospital discharges in 2003. For more information, visit the HCUP User Support Website at http://www.hcup-us.ahrq.gov or contact the HCUP Central Distributor (see below).

State Ambulatory Surgery Databases (SASD)are outpatient databases from Data Organizations in participating HCUP States, which capture surgeries performed on the same day in which patients are admitted and released. The SASD contain the ambulatory surgery encounter abstracts in participating States, translated into a uniform format to facilitate multi-State comparisons and analyses.  All of the databases include abstracts from hospital-affiliated ambulatory surgery sites. Some contain the universe of ambulatory surgery encounter abstracts for that State, including records from both hospital-affiliated and freestanding surgery centers. Composition and completeness of data files may vary from State to State. For more information, visit the HCUP User Support Website at http://www.hcup-us.ahrq.gov or contact the HCUP Central Distributor (see below).

Kids' Inpatient Database (KID)is a unique database of hospital inpatient stays for children 18 years of age and younger. The 1997 and 2000 KID was specifically designed to permit researchers to study a broad range of conditions and procedures related to child health issues. The KID is created and released every three years. For more information, visit the HCUP User Support Website at http://www.hcup-us.ahrq.gov or contact the HCUP Central Distributor (see below).

HCUP CENTRAL DISTRIBUTOR

HCUP databases are available for purchase through the AHRQ-sponsored HCUP Central Distributor.  All years of the NIS and KID are released through the HCUP Central Distributor. In addition, many of the HCUP State Partners allow the public release of the HCUP State Inpatient Databases (SID) and State Ambulatory Surgery Databases (SASD) through the HCUP Central Distributor.  Application Kits for purchasing the HCUP databases are available online at http://www.hcup-us.ahrq.gov or contact the HCUP Central Distributor directly. Information on how to obtain uniformly-formatted HCUP files from States not participating in the HCUP Central Distributor is also available from the HCUP Central Distributor:

HCUP Central Distributor
Phone: (866) 556-4287 (toll-free)
FAX: (301) 628-3201
E-mail: hcup@s-3.com

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HCUP USER SUPPORT

HCUP User Support (HCUP-US) provides technical assistance to all HCUP users and is designed to facilitate the use of HCUP data, software tools, and products. The goals of this service are to increase awareness of the strengths and uses of HCUP data and to enhance the skills of individuals using the data for research, education, and policy analysis. A user-friendly Website for HCUP-US is located at http://www.hcup-us.ahrq.gov. This site includes links to information on how to purchase and understand the HCUP databases, as well as links to HCUP User Support Services and Frequently Asked Questions. For further information, consultants are available via both telephone and E-mail to help in planning analytic research and to offer advice about appropriate uses of HCUP data.

HCUPnet

HCUPnet is a Web-based query tool for identifying, tracking, analyzing, and comparing statistics on hospitals at the national, regional, and state level. With HCUPnet you have easy access to national statistics and trends and selected state statistics about hospital stays. HCUPnet guides you step-by-step to obtain the statistics you need. HCUPnet generates statistics using the Nationwide Inpatient Sample (NIS), the Kids' Inpatient Database (KID), and the State Inpatient Databases (SID) for those states that have agreed to participate. In addition, HCUPnet provides Quick Statistics – ready-to-use tables on commonly requested information – and national statistics based on the AHRQ Quality Indicators. HCUPnet can be found at: http://hcupnet.ahrq.gov/.

TOOLS

AHRQ Quality Indicators (QIs)are clinical performance measures for use with readily available inpatient data. Methods and software for the AHRQ Quality Indicators can be downloaded from http://www.qualityindicators.ahrq.gov.

Clinical Classifications Software (CCS), formerly known as the Clinical Classifications for Health Policy Research (CCHPRs), are classification systems that group ICD-9-CM diagnoses and procedures into a limited number of clinically meaningful categories. Methods and software can be downloaded from the HCUP User Support Website, Tools and Software page, at http://www.hcup-us.ahrq.gov/tools_software.jsp.

Comorbidity Software assigns variables that identify comorbidities in hospital discharge records using ICD-9-CM diagnosis codes (International Classification of Diseases, Ninth Revision, Clinical Modification). Methods and software can be downloaded from the HCUP User Support Website, Tools and Software page, at http://www.hcup-us.ahrq.gov/tools_software.jsp.

Procedure Classes identify whether a procedure is (a) diagnostic or therapeutic, and (b) minor or major in terms of invasiveness and/or resource use. Software can be found at http://www.hcup-us.ahrq.gov/tools_software.jsp.

PUBLICATIONS

HCUP Research Notes and Fact Books report aggregate statistics and detailed analyses using HCUP data. To request copies, contact the AHRQ Publications Clearinghouse at (800) 358-9295 or send a postcard to: AHRQ Publications Clearinghouse, P.O. Box 8547, Silver Spring, MD 20907 or visit http://www.hcup-us.ahrq.gov/reports.jsp.

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Internet Citation: 2003 Introduction to the NIS. Healthcare Cost and Utilization Project (HCUP). July 2016. Agency for Healthcare Research and Quality, Rockville, MD. www.hcup-us.ahrq.gov/db/nation/nis/NIS_Introduction_2003_v7.jsp.
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