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National Hospital Discharge Survey (NHDS) Session Overview NHDS: Overview Carol DeFrances, Ph.D. Analytic Issues Jean Kozak, Ph.D. Examples of Research Marni Hall, Ph.D. Accessing Data Maria Owings, Ph.D. New Directions Bob Pokras

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National Hospital Discharge Survey (NHDS)

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session overview

Session Overview

NHDS: Overview

Carol DeFrances, Ph.D.

Analytic Issues

Jean Kozak, Ph.D.

Examples of Research

Marni Hall, Ph.D.

Accessing Data

Maria Owings, Ph.D.

New Directions

Bob Pokras

nhds survey years
NHDS Survey Years
  • Conducted annually 1965-present:
    • Latest data available: 2002
    • 2003 will be available this Winter
nhds survey design
NHDS Survey Design
  • National probability sample:
    • Short stay non-Federal hospitals
  • Three stage design:

1. Geographic units

2. Hospital

3. Discharge

more information on nhds design and operations
More Information on NHDS Design and Operations
  • NCHS Publications: Series 1, No. 39

“Design and Operation of the National Hospital Discharge Survey: 1988 Redesign”

nhds sample size
NHDS Sample Size
  • Hospitals:
    • About 500 hospitals sampled per year
  • Discharges:
    • Over 300,000 sampled per year
nhds data collection
NHDS Data Collection
  • Manual hospitals - 60%
  • Automated hospitals - 40%
nhds manual data collection
NHDS Manual Data Collection
  • NCHS – Statistical Design
  • Census Bureau – Field Work
  • Contractor – Coding and Data Entry
nhds automated data collection
NHDS Automated Data Collection
  • Electronic files obtained from:
    • States
    • Commercial firms
    • Individual hospitals
nhds data processing
NHDS Data Processing
  • NCHS:
    • Editing
    • Estimation
nhds estimation
NHDS Estimation
  • Weight:
    • Inverse of the probability of selection
    • Adjustments for non-response
    • Population weighting ratio adjustment
patient data
Patient Data
  • Age
  • Sex
  • Race
  • Expected source of payment
  • Discharge status
  • Marital status
hospital characteristics
Hospital Characteristics
  • Geographic region
  • Bed size
  • Ownership
medical data
Medical Data
  • Diagnoses and procedures
  • International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9-CM)
additional variables
Additional Variables
  • Days of care
  • Month of discharge
  • DRG
  • Weight
new variables for nhds
New Variables for NHDS

Available beginning in the 2001 NHDS:

- Source of Admission

- Type of Admission

national survey of ambulatory surgery nsas
National Survey of Ambulatory Surgery (NSAS)
  • Survey Years: Annually 1994-1996
  • Collected data on ambulatory (outpatient)

surgery in the U.S.

nsas design
NSAS Design
  • National probability sample
    • Short stay non-Federal hospitals (418)
    • Freestanding ambulatory surgery centers (333)
  • Sample size:
    • 120,000 sampled visits per year
more information on nsas design and operations
More Information on NSAS Design and Operations
  • Survey Design and Operations:

NCHS Publications: Series 1, No. 37

“Plan and Operation of the National Survey of Ambulatory Surgery”

additional variables collected for nsas
Additional Variables Collected for NSAS
  • Hospital vs. freestanding center
  • Type of anesthesia
  • Anesthesia provider
analytic issues

Analytic Issues

Lola Jean Kozak, Ph.D.

  • Utilization measures
  • Medical coding system
  • Statistical issues
nhds provides data on

NHDS provides data on


not people

measures include
Measures Include:
  • Discharges
  • Days of care
  • Average length of stay
  • Diagnoses
  • Surgeries/procedures
  • Include deaths
  • Include transfers to other hospitals or long-term care facilities
  • Do not usually include newborn infants
days of care
Days of Care
  • Total number of days discharged patients spend in the hospital
  • All stays are counted as at least 1 day.
  • The admission day is counted, but not the discharge day
  • The number of days divided by the number of discharges is the average length of stay.
  • Disease, injury or other reason for hospitalization
  • Coded according to US adaptations of the

International Classification of Diseases (ICD-9-CM)

  • Principal diagnosis: chiefly responsible for hospitalization
  • First-listed diagnosis: principal if specified, otherwise one listed first
  • All-listed: total number of times diagnoses appears on record
  • Any-listed: discharges with diagnosis in any position on record
hospital discharges with fractures 2002
Hospital discharges with fractures, 2002




Principal or first listed

Any listed

All listed

surgery procedures
  • Surgical (appendectomy)
  • Diagnostic (spinal tap)
  • Therapeutic (chemotherapy) procedures
  • Coded according to US adaptations of the International Classification of Diseases
nhds provides data on35

NHDS provides data on

inpatient procedures

not total procedures

versions of the international classification of diseases
Versions of the International Classification of Diseases
  • 8th revision used 1970-78
  • 9th revision used 1979-2004
  • 10th revision for use in future
8 th revision
8th Revision
  • Some codes different than in 9th Revision
  • Did not use E-codes
  • Made modifications in coding to accommodate available data
9 th revision
9th Revision
  • Addenda added annually since 1986
  • Codes added, deleted, expanded, and revised
  • Lists of changes available in annual summary reports, file documentation
  • Must use weighted data to obtain unbiased national estimates.
  • Each record has a weight
  • Sum the weights of the records
  • To be reliable, estimates must be based on at least 30 records .
  • And have a relative standard error of less than 30 percent.
  • Estimates based on 30-59 records should be used with caution.
standard errors
Standard Errors
  • Some standard errors are in Advance Data summaries
  • Generalized error curves are in the Series 13 Annual Summaries and data documentation
  • Use SUDAAN for specific standard errors - need access to confidential data

65 and over



< 15





Percent distribution of hospital inpatients by age

hospital transfers to long term care facilities in the 1990 s

Hospital Transfers to Long Term Care Facilities in the 1990’s

Jean Kozak

Long-Term Care Interface – June 2002

examples of health services research
Examples ofhealth services research:
  • Long-stay patients in short-stay hospitalsAcademyHealth Annual Meeting Presentation - Marni Hall and Maria Owings - June, 2003
  • Admit source and type

NAHDO Annual Meeting Presentation - Maria Owings and Marni Hall - December 2003

  • Differences in length of stay in Veterans Health Administration and NHDS hospitalsMedical Care - GE Rosenthal et al. - August 2003
common nhds research topics
Common NHDS Research Topics:
  • Deliveries
    • Type of birth, e.g. cesarean section
    • Obstetrical procedures
    • Length of stay
u s trends in obstetric procedures 1990 2000

U.S. Trends in Obstetric Procedures, 1990-2000

Jean Kozak and Julie Weeks

Birth – September 2002

common nhds research topics con d
Common NHDS Research Topics (con’d):
  • Newborns
    • Premature
    • Sick/well – common diagnoses
    • Length of stay in hospital
recent diagnoses studied

Recent diagnoses studied

Heart failureAtrial fibrillationArthritisPneumoniaMental disorders in childrenSepsis

recent procedures studied

Recent procedures studied:

Hip replacementsKnee replacementsCoronary artery revascularization proceduresCervical diskectomy


Example of trend analyses related to public health

Chartbook on Trends in Injury Hospitalizations

United States, 1979-2001

Melissa Heinen, Marni Hall, Manon Boudreault,

Lois Fingerhut

NCHS Publication

Available in the Fall of 2004

rate of hospitalization for hip fracture for females 65 and over 1980 2001

85 and over



Data are plotted on the log scale.

Rate of hospitalization for hip fracture for females 65 and over, 1980-2001
other public health medical care research

Other Public Health/Medical Care Research

DiabetesAsthmaObesityHypertensionChronic Obstructive Pulmonary Disease (COPD)


Other publications using NHDS trend data:

Health Care in America. Trends in Utilization,


Health United States, 2003

Healthy People 2010

for more information see

For more information see:

Internet Resources: Hospital Discharge and Ambulatory Surgery Data (handout)

what type of data are available
What type of data are available?
  • Publications, including annual reports
  • Data tabulations on selected topics
  • Public-use data files (micro-data)
how can i obtain data from the national hospital discharge survey
How Can I Obtain Data from theNational Hospital Discharge Survey?
  • Go to the Hospital Discharge and Ambulatory Surgery homepage on the Web:
  • Telephone the Hospital Care Statistics Branch: 301-458-4321
  • Contact NCHS Information Dissemination Staff 1-866-441-NCHS (toll free)
  • Send an email to:
annual publications from hospital care statistics branch
Annual Publications fromHospital Care Statistics Branch
  • ADVANCE DATA reports:
    • Provide early release of NHDS data
    • Are very general and usually short
    • 2002:
  • VITAL AND HEALTH STATISTICS Series 13 reports, e.g. Annual Summary and Detailed Reports
    • Provide more comprehensive statistics on utilization
    • Contain detailed tables of diagnoses and procedures
    • 2001:


navigating the nchs website
Navigating the NCHS Website
  • Several avenues are available to find data and reports on NHDS and NSAS
  • Know what you are looking for, e.g.
    • Microdata files & documentation
    • Published reports & tabulations
  • Follow links – many lead to the same place
what to know to access data and publications on the web
What to Know to Access Data and Publications on the Web
  • Publications, data tables, and data file documentation are in Adobe Acrobat PDF format.
  • Require use of the free Adobe Acrobat Reader software, available for download at
  • Data files are in ASCII format and must be downloaded via FTP server
  • After downloading, files must be “unzipped” using WINZIP or other data extraction software, available for free
links from the nchs homepage www cdc gov nchs
Links from the NCHS
  • Surveys & Data Collection Systems
    • National Health Care Survey, e.g.


  • Publications & Information Products
    • Published reports & Data Warehouse
  • Micro-data Access
    • Public Use Data Files & Documentation
nhds nsas homepage
NHDS & NSAS Homepage


provides links to all aspects of survey design, public use data, and dissemination products, including

    • Survey Description & Methodology
    • New Releases & Fact Sheets
    • NCHS Health E-Stats
    • Data Highlights & Selected Tables
    • Publications: reports and journal articles
    • Public Use Data Files (microdata)
links to nhds and nsas publications on the web
Links to NHDS and NSAS Publications on the Web


Lists annual publications and special topic reports by name and number (back to 1990 only)


Lists electronic products available for entire Center, including NHDS & NSAS products

  • AD & Series 13 reports cataloged by number,

not name (some back to 1960’s)

  • Links to Data Warehouse
data warehouse
Data Warehouse

Provides links by topic area for all NCHS products

(not just NHDS & NSAS), including

      • Public Use Files & Documentation
      • Published Reports, by type

e.g. Advance Data, Vital and Health Statistics Series 13 , etc (need to know report number)

      • ICD-9-CM coding index

ftpdata.htm[ FTP server ]

public use files available on the web
Public-Use Files Available on the Web
  • Data and documentation available for free from the NCHS website
    • NHDS: 1996 through 2002
    • NSAS: 1994, 1995, 1996
  • Files are “raw” ASCII data that require the use of standard statistical software packages, such as SAS, SPSS, Stata, etc.
what to know to access public use files on the web
What to Know to Access Public-Use Files on the Web
  • Downloadable public-use data files are “zipped” for a speedier download
  • “Unzip” these files with
    • WinZip (
    • PKunzip (
  • Data file documentation is available in PDF format
public use files available on cd rom
Public-Use Files Available on CD-ROM
  • Single year files available for some years

-- From NCHS, years 1990, 1994 to 1998

-- From NTIS, years 1970 to 1998

  • Two separate multi-year files containing
    • 1979-2002 data years (ICD-9-CM coding)
    • 1970-1978 data years (ICD-8 coding)

IMPORTANT: DRGs are available on single year files only. Multi-year files do NOT have DRGs.

how to get pu files on cd rom
How to Get PU Files on CD-ROM
  • FREE from NCHS by contacting
    • NCHS Information Dissemination Staff :

301-458-INFO or 1-866-441-NCHS

    • Hospital Care Statistics Branch: 301-458-4321
  • Purchased from National Technical Information Service (NTIS) *
    • phone 1-800-553-6847 or (703) 605-6000
    • online at

* Annual files for single years prior to 1994 can only be obtained from NTIS (not available from NCHS)

icd 9 cm
  • For full-text, addenda, and conversion tables of ICD-9-CM, see
  • Full-text ICD-9-CM documents are RTF (Rich Text Format) files and can be handled with any word processing package.
  • Addenda and conversion tables are PDF documents.
restricted data in nhds
Restricted Data in NHDS
  • HCSB maintains confidential information in files which are not publicly disseminated
  • Restricted data pose disclosure risks to survey respondents
  • These data are available to researchers through the NCHS Research Data Center (RDC)
through the rdc researchers can use
Through the RDCResearchers Can Use:
  • Confidential files for NHDS and NSAS variance estimation
  • NHDS and NSAS analytic files that have been linked with outside data sources
confidential variables available only on restricted files
Confidential Variables Available Only on Restricted Files
  • ZIPCODE for residence of discharged patient
  • ZIPCODE for hospital
  • STATE/COUNTY FIPS CODE for both patient and hospital
  • AHA ID for hospital


NOTE: Identifiers such as patient name, address, and SSN are not collected in the NHDS

nhds can be linked with other files
NHDS can be linked with other files
  • NHDS + American Hospital Association (AHA)
  • NHDS + Area Resource File (ARF)
  • Linkage is with contextual NOT personal / demographic information
  • Contextual data include
    • Hospital characteristics, services (AHA)
    • County level information (ARF)
beyond 20 20 browser
Beyond 20/20 Browser
  • Database providing up-to-date information on national trends and key variables that depict the health status of older Americans
  • Data for persons 45 years old and over by sex and race
nhds or nsas questions
NHDS or NSAS Questions?

Phone: 301-458-4321

Fax: 301-458-4032


new directions
New Directions
  • Internet Data
    • Beyond 20/20
    • Improved internet links
  • Linked files
new directions87
New Directions
  • Clinical data
  • Evaluation of drugs in the NHDS
nsas and nhds
  • NSAS status
  • NHDS status
nhds and nsas


Carol DeFrances

Jean Kozak

Marni Hall

Maria Owings

Bob Pokras