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Overview to the HCUP Nationwide Inpatient Sample (NIS), 2001

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

Sponsored by the Agency for Healthcare Research and Quality

 

 

OVERVIEW TO

THE HCUP NATIONWIDE INPATIENT SAMPLE (NIS),

2001

 

 

These pages provide only an introductory overview to the NIS package.

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

 

Issued July 2003

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
Social & Scientific Systems, Inc.

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.


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) website 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 website.

HCUP User Support:

Information about the content of the HCUP databases is available on the HCUP User Support (HCUP-US) website (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 2001
NATIONWIDE INPATIENT SAMPLE (NIS)?

  • Five States have joined the NIS in 2001: Michigan, Minnesota, Nebraska, Rhode Island, and Vermont.

  • We provide a brief section in this Overview on "How to Use the NIS."

  • The 1988 - 2001 NIS are available for purchase through the HCUP Central Distributor sponsored by the Agency for Healthcare Research and Quality:
    • HCUP Central Distributor
      Social & Scientific Systems, Inc.
      Phone: (866) 556-4287 (toll-free)
      FAX: (301) 628-3201
      E-mail: hcup@s-3.com

    The NIS is no longer available through the National Technical Information Service (NTIS).
  • HCUP User Support (HCUP-US) is available online through the AHRQ-sponsored HCUP Website at http://www.hcup-us.ahrq.gov. This Website includes information on HCUP databases, tools, software, reports, and technical assistance.

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 users for
purchasing the HCUP National Inpatient Sample (NIS).



HCUP National 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 healthcare 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 1000 hospitals sampled to approximate a 20-percent stratified sample of U.S. community hospitals. The NIS is available for a 14-year time period, from 1988 to 2001, allowing analysis of trends over time.

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 children.

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.

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

INTRODUCTION TO THE HCUP NATIONWIDE INPATIENT SAMPLE ( NIS )

OVERVIEW OF NIS DATA

The Nationwide Inpatient Sample 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 2001 NIS Documentation CD-ROM and on the AHRQ-sponsored HCUP Website at http://www.hcup-us.ahrq.gov. 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 Sample found on 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
bracket spanning the years 1993-2001 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

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, 1988-2001
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

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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 - three types of fixed-width ASCII formatted data files are included in the NIS:

    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.

    Subsample Inpatient Core Files: Each of these discharge-level files contain 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.

    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.

  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 6.

    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.

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 identify the data elements that can be found in the inpatient stay and hospital weights files, respectively. 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, Starting in 1998

Note: Beginning in 1998, the NIS differs from previous NIS releases; some data elements were dropped, some added, and for some data elements the values were changed.

Data Element Description (numbers in brackets indicate variable coding)
AGE Age in years at admission
AGEDAY Age in days (coded only when the age in years is less than 1) at admission
AMONTH Admission month
ASOURCE Admission source: (1) ER, (2) another hospital, (3) another facility including long-term care, (4) court/law enforcement, (5) routine/birth/other
ASOURCE_X Admission source, as received from data source. Available beginning in 1998. *
ATYPE Admission type: (1) emergency, (2) urgent, (3) elective, (4) newborn, (6) other
AWEEKEND Admission on weekend: (0) admission on Monday-Friday, (1) admission on Saturday-Sunday. AWEEKEND is available beginning in 1998. In 1988-1997, the data element admission day of week (ADAYWK) is available.
DIED Indicates in-hospital death: (0) did not die during hospitalization, (1) died during hospitalization
DISCWT Discharge weight on Core file and Hospital Weights file. 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. Discharge weights to the sample frame and state are available only in 1988-1997
DISCWT10 Discharge weight on 10% subsample file. 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.
DISCWTcharge Discharge weight for national estimates of total charges on Core file and Hospital Weights file. Only available in 2000.
DISCWTcharge10 Discharge weight for national estimates of total charges on 10% subsample file. Only available in 2000.
DISPUB92 Disposition of patient (discharge status), UB92 coding: (1) routine, (2) short term hospital, (3) skilled nursing facility, (4) intermediate care, (5) another type of facility, (6) home healthcare, (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, (50) Hospice, home, (51) Hospice, medical facility, (61) hospital-based Medicare approved swing bed, (62) another rehabilitation facility, (63) long term care hospital, (71) another institution for outpatient services, (73) this institution for outpatient services, (99) discharged alive, destination unknown. Less detail is available in the data element DISPUniform (beginning in 1998) and in DISP (from 1988-1997).
DISPUniform Disposition of patient (discharge status), uniform coding: (1) routine, (2) transfer to short term hospital, (5) other transfers, including skilled nursing facility, intermediate care, and another type of facility, (6) home healthcare, (7) against medical advice, (20) died in hospital, (99) discharged alive, destination unknown
DQTR Discharge quarter
DRG Diagnosis Related Group (DRG) in use on discharge date
DRG10 DRG Version 10 (effective October 1992 - September 1993). Discontinued in 2000.
DRG18 DRG Version 18 (effective October 2000 - September 2001). Available beginning in 1998.
DRGVER Grouper version in use on discharge date
DSHOSPID Hospital number as received from the data source
DX1-DX15 Principal and secondary diagnoses
DXCCS1-DXCCS15 Clinical Classifications Software (CCS) category for all diagnoses
FEMALE Gender of patient: (0) male, (1) female
HOSPID HCUP hospital number (links to Hospital Weights file)
HOSPST State postal code for hospital (e.g., AZ for Arizona)
HOSPSTCO Modified Federal Information Processing Standards (FIPS) State/county code for hospital, links to Area Resource File (available from the Bureau of Health Professions, Health Resources and Services Administration)
KEY Unique record number
LOS Length of stay, edited
LOS_X Length of stay, as received from data source
MDC Major Diagnosis Category (MDC) in use on discharge date
MDC10 MDC Version 10 (effective October 1992 - September 1993). Discontinued in 2000.
MDC18 MDC Version 18 (effective October 2000 - September 2001). Available beginning in 1998.
MDID_S Synthetic attending physician number, available prior to 2001. This data element was renamed MDNUM1_S beginning in 2001
MDNUM1_S Synthetic attending physician number . This data element was called MDID_S prior to 2001 and was renamed beginning in 2001.
MDNUM2_S Synthetic secondary physician number. This data element was called SURGID_S prior to 2001 and was renamed beginning in 2001
NDX Number of diagnoses coded on the original record
NEOMAT Neonatal/maternal flag: (0) not maternal or neonatal, (1) maternal diagnosis or procedure, (2) neonatal diagnosis, (3) maternal and neonatal on same record
NIS_STRATUM Stratum used to sample hospitals, based on geographic region, control, location/teaching status, and bed size
NPR Number of procedures coded on the original record
PAY1 Expected primary payer, uniform: (1) Medicare, (2) Medicaid, (3) private including HMO, (4) self-pay, (5) no charge, (6) other
PAY1_X Expected primary payer, as received from the data source. Available beginning in 1998. *
PAY2 Expected secondary payer, uniform: (1) Medicare, (2) Medicaid, (3) private including HMO, (4) self-pay, (5) no charge, (6) other
PAY2_X Expected secondary payer, as received from the data source. Available beginning in 1998. *
PR1-PR15 Principal and secondary procedures
PRCCS1-PRCCS15 Clinical Classifications Software (CCS) for all procedures
PRDAY1-PRDAY15 For each procedure, the number of days from admission. Day of secondary procedures available beginning in 1998
RACE Race includes (1) white, (2) black, (3) Hispanic, (4) Asian or Pacific Islander, (5) Native American, (6) other. This data element is not available in all states.
SURGID_S Synthetic second physician number, available prior to 2001. This is often the primary surgeon. This data element was renamed MDNUM2_S beginning in 2001.
TOTCHG Total charges, edited
TOTCHG_X Total charges, as received from data source
YEAR Calendar year
ZIPINC Median household income for patient's ZIP Code: (1) $1-$24,999, (2) $25,000-$34,999, (3) $35,000-$44,999, (4) $45,000 and above
*For categorical data elements with _X suffix, see Description of Data Elements (on the NIS Documentation CD-ROM) for state-specific coding.

Table 4. Data Elements in the NIS Hospital Weights File, Starting in 1998

Note: Beginning in 1998, the NIS differs from previous NIS releases; some data elements were dropped, some added, and for some data elements the values were changed.

Data Element Description (numbers in brackets indicate variable coding)
AHAID AHA hospital identifier that matches AHA Annual Survey of Hospitals (not available for all states)
DISCWT Discharge weight on Core file and Hospital Weights file. 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. Discharge weights to the sample frame and state are available only in 1988-1997.
DISCWTcharge Discharge weight for national estimates of total charges on Core file and Hospital Weights file. Only available in 2000.
HOSPADDR Hospital address from AHA Survey (not available for all states)
HOSPCITY Hospital city from AHA Survey (not available for all states)
HOSPID HCUP hospital number (links to inpatient Core files)
HOSPNAME Hospital name from AHA Survey (not available for all states)
HOSPST Hospital state postal code for hospital (e.g., AZ for Arizona)
HOSPWT Weight to hospitals in AHA universe
HOSPZIP Hospital zip code from AHA Survey (not available for all states)
HOSP_BEDSIZE Bed size of hospital: (1) small, (2) medium, (3) large
HOSP_CONTROL 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
HOSP_LOCATION Location: (0) rural, (1) urban
HOSP_LOCTEACH Location/teaching status of hospital: (1) rural, (2) urban non-teaching, (3) urban teaching
HOSP_REGION Region of hospital: (1) Northeast, (2) Midwest, (3) South, (4) West
HOSP_TEACH Teaching status of hospital: (0) non-teaching, (1) teaching
IDNUMBER AHA hospital identifier without the leading 6 (not available for all states)
NIS_STRATUM Stratum used to sample hospitals; includes geographic region, control, location/teaching status, and bed size
N_DISC_U Number of AHA universe discharges in NIS_STRATUM
N_HOSP_U Number of AHA universe hospitals in NIS_STRATUM
S_DISC_U Number of sample discharges in NIS_STRATUM
S_HOSP_U Number of sample hospitals in NIS_STRATUM
TOTAL_DISC Total number of discharges from this hospital in the NIS
YEAR Calendar year

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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 5. 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 NIS Documentation CD-ROM.

GETTING STARTED

NIS information is provided on two CD-ROMs. The NIS data files are on CD-ROM #1 and the NIS documentation and tools are on CD-ROM #2.

NIS Data Files

In order to load NIS data onto your PC, you will need 5 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 95 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/. Exit Disclaimer For further assistance in installing and running the Adobe Acrobat Reader on your computer platform, please consult your local support personnel.

Table 6 describes the documentation and tools 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 Website at http://www.hcup-us.ahrq.gov.

Table 6. NIS Documentation CD-ROM
Directory Description
Root Includes:
  • DOCUMENTATION.README.TXT file with introductory information on accessing the NIS documentation
/General Information Includes:
  • Overview of the NIS (PDF file)
  • Sources of NIS Data and State-Specific Restrictions (PDF file)
  • Data Use Agreement for the Nationwide Inpatient Sample (PDF file)
/Special Reports Includes:
  • Design of the Nationwide Inpatient Sample (PDF file)
  • Changes in NIS Sampling and Weighting Strategy for 1998 (PDF file)
  • NIS Comparison Report, 2000 (PDF file)
  • Calculating Variances using Data from the HCUP Nationwide Inpatient Sample, 2001 (PDF file)
  • HCUP Coding Practices (PDF file)
  • HCUP Quality Control Procedures (PDF file)
  • HCUP Hospital Identifiers (PDF file)
/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, and Hospital Weights. (Text files)
/Description of Data Elements Includes information on all NIS variables such as uniform coding and state-specific information. One file per data type: Core and Hospital Weights. (PDF files)
/Summary Statistics Includes summary statistics (means and frequencies) on NIS data. One file per data file: Core, Core Subsample #1, Core Subsample #2, and Hospital Weights. (PDF files)
/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, and Hospital Weights. (Text files)
/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, and Hospital Weights. (Text files)
/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)
/Adobe Acrobat Reader Adobe Acrobat Reader files for IBM compatible for Microsoft Windows 95 or higher. (One text, one HTML, and one application file)

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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
  • DISCWT for all analyses
  • DISCWT10 for all analyses
2000
  • DISCWT to create nationwide estimates for all analyses except those that involve total charges.
  • DISCWT10 to create nationwide estimates for all analyses except those that involve total charges.
  • DISCWTCHARGE to create nationwide estimates of 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

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.

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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 1000 hospitals sampled to approximate a 20-percent stratified sample of U.S. community hospitals. The NIS is available for a 14-year time period, from 1988 to 2001, 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 80 percent of all U.S. community hospital discharges. 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 200 KID was specifically designed to permit researchers to study a broad range of conditions and procedures related to child health issues. 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 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. HCUPnet can be found at: https://datatools.ahrq.gov/hcupnet.

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/toolssoftware/ccs/ccs.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 http://www.hcup-us.ahrq.gov/toolssoftware/comorbidity/comorbidity.jsp.

PUBLICATIONS

HCUP Research Notes 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 the AHRQ Website https://www.ahrq.gov/research/data/hcup/index.html.

DATA USE AGREEMENT FOR THE NATIONWIDE INPATIENT SAMPLE

This agreement must be signed by anyone seeking to use data in the Nationwide Inpatient Sample (NIS) maintained by the Center for Organization and Delivery Studies (CODS), Agency for Healthcare Research and Quality (AHRQ) before access to such data can be granted. All data maintained by CODS/ AHRQ is confidential or proprietary except data specified for restricted access public release, or data authorized by AHRQ and the original data source for re-release.

Under section 924(c) of the Public Health Service Act (42 U.S.C. 299c-3(c)), data that identifies individuals or establishments collected by the Agency for Healthcare Research and Quality (AHRQ) may be used only for the purpose for which they were collected. Data supplied to AHRQ under the auspices of HCUP were provided by the data sources only for research, analysis, and aggregate statistical reporting.

No identification of persons--Any effort to determine the identity of any person contained in the databases (including but not limited to patients, physicians, and other healthcare providers) or to use the information for any purpose other than for research, analysis, and aggregate statistical reporting would violate the conditions of this data use agreement and therefore the above-referenced AHRQ confidentiality statute. Furthermore, under the statute, no identifying information may be published or released in any way without the consent of the person who supplied the information or who can be identified by the information. AHRQ omits from the data set all direct personal identifiers, as well as characteristics that might lead to identification of persons. It may be possible in rare instances, through complex analysis and with outside information, to ascertain from the data sets the identity of particular persons. Considerable harm could ensue if this were done. By virtue of this agreement, the undersigned agrees that such attempts will be prohibited and that information which could identify individuals directly or by inference will not be released or published. Because of these restrictions, users of the data must agree that they will not attempt to contact individuals for the purpose of verifying information supplied in the HCUP databases. Any questions about the data must be referred to AHRQ only.

Use of Establishment identifiers--Section 924(c) of the Public Health Service Act (42 U.S.C. 299c-3(c) ) also restricts the use of any information that allows the identification of establishments to the purpose for which the information was collected. Permission was obtained from the data sources (state data organizations, hospital associations, and data consortia) to use the identification of hospitals (when such identification appears in the data sets) for the purpose of conducting research only. Such research purpose includes linking institutional information from outside data sets for analysis and aggregate statistical reporting. Such purpose does not include the use of information in the data sets concerning individual establishments for commercial or competitive purposes involving those individual establishments, or to determine the rights, benefits, or privileges of establishments. Users of the data must not identify establishments directly or by inference in disseminated material. In addition, users of the data must not contact establishments for the purpose of verifying information supplied in the HCUP databases. Any questions about the data must be referred to AHRQ only.

The undersigned gives the following assurances with respect to the AHRQ data sets.

I understand that these assurances are collected for the United States Agency for Healthcare Research and Quality to require compliance with its statutory confidentiality requirement. My signature indicates my agreement to comply with the above-stated requirements with the knowledge that any violation of this statute is subject to a civil penalty of up to $10,000 under 42 U.S.C. 299c-3(d), and that deliberately making a false statement about this or any matter within the jurisdiction of any department or agency of the Federal Government violates 18 U.S.C. 1001 and is punishable by a fine of up to $10,000 or up to five years in prison. Violators of this agreement may also be subject to penalties under state confidentiality statutes that apply to these data for particular states.

 
Signed:__________________________________________________________________ Date:_________________________
Print or Type Name:_______________________________________________________________________
Title:__________________________________________________________________________________________________
Organization:____________________________________________________________________________________________
Address:________________________________________________________________________________________________
City:______________________________________________________ State:__________ ZIP Code:____________________
Phone Number:______________________________________________ Fax: ________________________________________
E-mail:________________________________________________________________________________________________

Note to Purchaser: Shipment of the data product will only be made to the person who signs this data use agreement.

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Internet Citation: 2001 NIS Introduction. 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_2001.jsp.
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