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National Hospital Discharge Survey (NHDS). National Survey of Ambulatory Surgery (NSAS). Centers for Disease Control and Prevention. Session Overview. NHDS and NSAS: Overview Bob Pokras Analytic Issues Jean Kozak, Ph.D. Examples of Research Marni Hall, Ph.D. Accessing Data

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national hospital discharge survey nhds

National Hospital Discharge Survey(NHDS)

National Survey of Ambulatory Surgery


Centers for Disease Control and Prevention

session overview

Session Overview

NHDS and NSAS: Overview

Bob Pokras

Analytic Issues

Jean Kozak, Ph.D.

Examples of Research

Marni Hall, Ph.D.

Accessing Data

Maria Owings, Ph.D.

New Directions

national health care surveys
National Health Care Surveys
  • Visits to
    • Doctors’ offices (NAMCS)
    • Emergency rooms (NHAMCS)
    • Outpatient departments (NHAMCS)
  • Inpatients (NHDS)
  • Ambulatory surgery (NSAS) (1994-96)
  • Long term care
    • Nursing homes (NNHS)
    • Home health care (NHHCS)
    • Hospices (NHHCS)


Internet Resources: Hospital Discharge and Ambulatory Surgery Data

For an email of this table of hotlinks, write to:

survey years
Survey Years
  • NHDS: Annually 1965-present
    • Latest data available – 2000
    • 2001 will be available this Winter
  • NSAS: Annually 1994-1996
survey design and operations
Survey Design and Operations
  • NCHS Publications
    • NSAS – Series 1 No. 37
    • NHDS – Series 1 No. 39
survey design
Survey Design
  • Similar designs and methods
  • National probability samples
    • Short stay non-Federal hospitals (NHDS/NSAS)
    • Freestanding ambulatory surgery centers (NSAS)
survey design13
Survey Design
  • Three stage design
  • PSU
    • Facility
      • Discharge/visit
facility sample size
Facility Sample Size
  • 525 NHDS hospitals
  • 751 NSAS facilities
    • 418 Hospitals
    • 333 Freestanding surgery centers
response rates
Response Rates
  • NHDS – over 90 percent

300,000 sampled discharges per year

  • NSAS -- 80 percent for hospitals

-- 70 percent for FSASC

120,000 sampled visits per year

data collection
Data Collection
  • NHDS
    • Manual; 60%
    • Automated; 40%
  • NSAS
    • All manual
manual data collection
Manual Data Collection
  • NCHS – Statistical Design
  • Census Bureau – Field Work
  • ASI – Coding and Data Entry
automated data collection
Automated Data Collection
  • Purchase files
    • States
    • Commercial firms
    • Individual hospitals
data collection19
Data Collection
  • NCHS
    • Editing
    • 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
facility characteristics
Facility Characteristics
  • Geographic region
  • Bed size (NHDS)
  • Ownership (NHDS)
  • Hospital vs. Freestanding (NSAS)
medical data
Medical Data
  • Diagnoses and procedures
  • International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9-CM)
additional variables
Additional Variables
  • NHDS
    • Days of care
    • Month of admission/discharge
    • DRG
  • NSAS
    • Month of visit
    • Type of anesthesia
    • Anesthesia provider
    • WEIGHT
new variables for nhds
New Variables for NHDS
  • Available for Year 2001 NHDS
    • Source of Admission
    • Type of Admission
source of admission
Source of Admission
  • Physician Referral
  • Clinical Referral
  • HMO Referral
  • Transfer from a Hospital
  • Transfer from Skilled Nursing Facility
  • Transfer from other health facility
  • Emergency Room
  • Court/Law Enforcement
  • Other
  • Not Available
type of admission
Type of Admission
  • Emergent
  • Urgent
  • Elective
  • Newborn
  • Not available/unknown
  • Source of the data
    • Design
    • Methods
  • Variables
  • Utilization measures
  • Populations
  • 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
average length of stay
Average Length of Stay
  • Calculated by dividing the number of days of care by the number of discharges
  • May want to examine length of stay distributions
  • Disease, injury or other reason for hospitalization
  • Coded according to US adaptations of the International Classification of Diseases
  • 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 2000
Hospital discharges with fractures, 2000




Principal or first listed

All listed

Any 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 on46

NHDS Provides Data on

Inpatient Procedures

Not Total Procedures

procedures mainly performed in inpatient settings 1996
Procedures mainly performed in inpatient settings, 1996


Coronary artery bypass graft




Number in thousands

procedures mainly performed in ambulatory settings 1996
Procedures mainly performed in ambulatory settings, 1996


Of knee

D & C

Endoscopy of

large intestine


of lens

Number in thousands

population for rates
Population for Rates
  • Mid-year population estimates from the U.S. Bureau of the Census
  • Civilian resident population
  • Adjustments for underenumeration
versions of the international classification of diseases
Versions of the International Classification of Diseases
  • 8th revision used 1970-78
  • 9th revision used 1979-2002
  • 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
reliable estimates
Reliable Estimates
  • Are based on 30 records or more*
  • And have a relative standard error of 30 percent or less

*Use estimates based on 30-59 records 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
hospital transfers to long term care facilities in the 1990 s

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

Lola Jean Kozak, Ph.D.

Long-Term Care Interface – June 2002

transfers to long term care 1990 1999
Transfers to long-term care,1990-1999

Number in Millions

2.8 Million

1.6 Million

hospital discharges transferred to long term care institutions by length of stay 1990 1999
Hospital discharges transferred to long-term care institutions by length of stay, 1990-1999

Hospital Stay

Transfers in Thousands

long term transfers by first listed diagnoses
Long-term transfers by first-listed diagnoses



Injury & poisoning



Endocrine, metabolic



Infectious & parasitic

Mental disorders









Number in thousands


hospital transfers to long term care institutions study
Hospital Transfers to Long Term Care Institutions study:
  • Trend data – 10 years
  • Changing roles of hospitals and nursing homes
  • Assessment of the effects of Medicare policy changes
  • Post-acute care in nursing homes substituting for end of hospital stay
trends in avoidable hospitalization united states 1980 1998

Trends in Avoidable Hospitalization: United States, 1980-1998

Lola Jean Kozak, Ph.D.

Margaret J. Hall, Ph.D.

and Maria F. Owings Ph.D.

avoidable hospitalization diagnoses
Avoidable hospitalization diagnoses
  • Selected by a panel of physicians
  • Can often be prevented, controlled, or managed over time without the need for hospitalization if the patient receives timely and appropriate ambulatory care
  • Used as indicators of access and the adequacy of ambulatory care
diagnoses studied in avoidable hospitalization study
Diagnoses studied in avoidable hospitalization study
  • Pneumonia
  • Congestive heart failure
  • Asthma
  • Cellulitis
  • Perforated or bleeding ulcer
  • Pyelonephritis
  • Diabetes with ketoacidosis or coma
  • Ruptured appendix
  • Malignant hypertension
  • Hypokalemia
  • Immunizable conditions
  • Gangrene
avoidable hospitalizations
Avoidable hospitalizations

1980 1998

  • # of discharges 2,200,000 3,700,000 million
  • Rate per 1,000

population 99.2 133.8

trend in rate of avoidable hospitalizations69
Trend in rate of avoidable hospitalizations

Over 65



Under 65



avoidable hospitalization study
Avoidable hospitalization study:
  • Trend data – 20 years
  • Avoidable hospitalization conditions as defined by the literature
  • Measured access to care over time
  • Identified disparities between elderly/nonelderly and white/black and identified those who should be targeted for intervention
  • Used as a model for additional research funded by Center for Medicare and Medicaid Services
pneumonia hospital discharge rate for the elderly
Pneumonia hospital discharge rate for the elderly


65 and over

Rate per 10,000


disparities in the rate of hospitalization for pneumonia patients in rural and urban areas
Disparities in the Rate of Hospitalization for Pneumonia Patients in Rural and Urban Areas

Maria F. Owings, Ph.D.

Margaret J. Hall, Ph.D.

study objectives
Study Objectives
  • To compare urban and rural patients hospitalized for pneumonia based on
    • Patient characteristics
    • County characteristics, including health services availability and socioeconomic status (SES)
disparity in urban rural pneumonia hospitalizations 2000
Disparity in Urban/Rural Pneumonia Hospitalizations, 2000



Rate per 10,000 population



indicators of severity of illness
Indicators of Severity of Illness


Average # diagnoses 5.1 4.9

% with serious

comorbidities 36% 40%

Average # seriouscomorbidities 1.2 1.31

1Significant Difference


Indicators of Severity of Illness


Average age 59 641

Average length of 6.2 5.31stay (days)

Routine Discharge 70% 65%

1Significant difference


Education, Unemployment

and Poverty

% w/some college

Unemployment rate

% in poverty


MD/Hospital Availability

Rate per 1,000 elderly

Active MD’s

Hospital Beds

what policies could reduce avoidable hospitalizations
What policies could reduce avoidable hospitalizations?
  • Promotion of rural managed care
  • Programs which attract/keep rural MD’s
  • More affordable, accessible outpatient health care
  • More health education / outreach programs - e.g. smoking cessation, influenza / pneumonia shots
urban rural pneumonia hospitalization study
Urban/rural pneumonia hospitalization study:
  • Urban/rural indicators
  • NHDS merged with Area Resource File (ARF) data
  • Severity of illness indicators using NHDS data
  • Policy recommendations
medical care expenditures for hypertension its complications and its comorbidities

Medical Care Expenditures for Hypertension, Its Complications, and Its Comorbidities

Thomas A. Hodgson, Ph.D. , NCHS

Liming Cai, Ph.D., NOVA Research Co.

estimated the economic burden of hypertension using utilization for
Estimated the economic burden of hypertension using utilization for:
  • First-listed hypertension
  • Cardiovascular complications
  • Unrelated conditions for which hypertensives are at greater risk
  • Comorbidities, i.e. secondary diagnoses
data from the centers for medicare and medicaid services cms
Data from the Centers for Medicare and Medicaid Services (CMS):
  • Personal Health Expenditures
  • Part B Data

Data from the Agency for Healthcare Research and Quality (AHRQ):

  • National Medical Expenditure Survey
data from the national center for health statistics
Data from the National Center for Health Statistics:
  • National Hospital Discharge Survey
  • National Ambulatory Medical Care Survey
  • National Hospital Ambulatory Medical Care Survey
  • National Home and Hospice Care Survey
  • National Nursing Home Survey
  • National Health Interview Survey
data on hospital costs were calculated using these data
Data on hospital costs were calculated using these data:
  • National Hospital Discharge Survey data on the number of inpatient days
  • National Medical Expenditure Survey data on the average facility charge per hospital inpatient day

Total hospital expenditures attributed to hypertension

$ 4.2 billion - diagnosis of hypertension

$ 17.1 billion - cardiovascular complications

$ 24.2 billion - other diagnoses


$45.5 billion – total hospital expenditures attributed to hypertension

expenditures for hypertension 1998
Expenditures for hypertension, 1998

Home health care


Nursing home


Prescription Drugs


Hospital care


Physician Services



Total expenditures attributed to hypertension

$ 22.8 billion - diagnosis of hypertension

$ 29.7 billion - cardiovascular complications

$ 56.4 billion - other diagnoses


$108.8 billion - total expenditures attributed to hypertension

expenditures for hypertension study
Expenditures for hypertension study:
  • Example of how cost data can be combined with utilization data
  • Hospital care studied as part of entire spectrum of health services – how it fits into the total picture
  • Uses multiple national data sources
  • Regression analyses
  • Provides data for cost benefit analysis
accessing data from nhds and nsas

Accessing Data from NHDS and NSAS

Maria Owings, Ph.D.

Centers for Disease Control and Prevention

sources of available data
Sources of Available Data
  • Publications, including annual reports
    • Downloadable from the Internet
      • Data years 1985 through 2000
    • Order and purchase – years before 1993
  • Data tables on selected topics –viewed or downloaded from Internet
  • Public-use data files for DO-IT-YOURSELF analysis
    • Downloadable from the Internet
    • On CD-ROM
  • ICD-9-CM – to assist in using medical data
quick and easy access to nhds and nsas data
Quick and Easy Access to NHDS and NSAS Data
  • Telephone the Hospital Care Statistics Branch: 301-458-4321
  • Send an email to:
  • Go to the NCHS website on the World Wide Web:
annual publications
Annual Publications
  • ADVANCE DATA on Vital and Health Statistics reports provide early release of NHDS data
    • Very general and usually short
  • Series 13 Reports provide more specific statistics on hospital utilization
    • Are more comprehensive and contain detailed tables of diagnoses and procedures
recent annual publications
Recent Annual Publications
  • 2000 NHDS Advance Data :
  • 1999 NHDS Annual Summary :
    • Includes estimates of diagnoses and procedures by detailed ICD-9-CM code number
what to know to access data and pubs on the www
What to Know to Access Data and Pubs on the WWW
  • Publications and data tables are in Adobe Acrobat PDF format.
  • Require use of the free Adobe Acrobat Reader software, available for download at
where to find nhds and nsas data and pubs on the www
Where to Find NHDS and NSAS Data and Pubs on the WWW

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

  • Provides links by topic area for all NCHS products (not just NHDS & NSAS), including
    • Data Warehouse (for microdata and tabulations)
    • Published Reports (by type, e.g. Advance Data, Series 13 Vital and Health Statistics, etc)

--ADs and Series 13 for pre-1990 years

nhds nsas homepage
NHDS & NSAS Homepage
  • provides links to all aspects of survey design, data, and dissemination, including
    • Survey Methodology and Data Collection
    • Publications and Journal Articles
    • Public Use Data Files (microdata)
    • Special Reports
    • NCHS Health E-Stats
    • Data Highlights & Selected Tables on topics such as hospital discharges among females with deliveries, HIV inpatients, newborn infants, and hospital inpatient deaths, and ambulatory surgery utilization
public use files available on the internet
Public-Use Files Available on the Internet
  • Data and documentation available for free from the NCHS website
    • NHDS: 1996 through 2000
    • NSAS: 1994, 1995, 1996
  • These are “raw” ASCII data that require the use of statistical software packages, such as SAS, SPSS, Stata, etc.
what to know to access public use files on the www
What to Know to Access Public-Use Files on the WWW
  • Downloadable public-use data files are “zipped” for a speedier download.
  • “Unzip” these files with
    • WinZip at
    • PKunzip at
  • Data documentation are available either as text files or PDF files.
public use files available on cd rom
Public-Use Files Available onCD-ROM
  • Two separate multi-year files containing
    • 1979-2000 data years (ICD-9-CM coding)
    • 1970-1978 data years (ICD-8 coding)
  • Single year files for 1990, 1994 to 2000
  • 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
  • Can be obtained at no cost from NCHS
    • Division of Data Services: 301-458-INFO
    • Hospital Care Statistics Branch: 301-458-4321
  • Or ordered from National Technical Information Service (NTIS)
    • by phone at1-800-553-6847 or (703) 605-6000
    • online at
  • Annual files for single years prior to 1994 can be ordered from NTIS, not directly 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 restricted from unauthorized use
  • 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
nchs research data center
NCHS Research Data Center
  • Located within NCHS facilities in Hyattsville, MD
  • Requires preapproval of research projects by an internal proposal review committee
  • Subjects analytic results to disclosure limitation review and clearance
  • Provides different modes of data access for approved research projects
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
  • DESIGN VARIABLES needed to run SUDAAN to obtain variances of complex NHDS statistics


  • NOTE: Patient name and address are NOT collected in the NHDS
restricted data needed by sudaan for nhds and nsas applications
Restricted Data Needed by SUDAAN for NHDS and NSAS Applications:
  • Variables corresponding to design stages for sampling and stratification
  • Population counts at each sampling stage
  • Type of sampling performed at each stage
sudaan software
SUDAAN Software
  • Incorporates design-related variables unique to each survey
  • Utilizes sampling weights of discharges and visits that reflect unequal probabilities of selection
  • Produces sampling errors for NHDS and NSAS estimates that take into account the complexity of the survey design
nhds linked files
NHDS Linked 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)
american hospital association aha database
American Hospital Association (AHA) Database
  • Hospital-specific data on over 6,200 hospitals and health care systems
  • More than 600 data items collected with the assistance of State and Metropolitan Hospital Associations
types of variables in aha
Types of Variables in AHA
  • Organizational Structure
  • Staffing
  • Utilization
  • Facilities and Services
  • Financial
  • Geographic codes
  • Approval and Accreditation Codes
area resource file arf
Area Resource File(ARF)
  • County-specific health resources information system designed to aid research on the health care delivery system and factors that may impact health status and health care in the U.S.
  • Contains more than 7,000 variables from over 50 different source files for each county.
general categories of variables in the arf
General Categories of Variables in the ARF
  • Health facilities
  • Health professions
  • Health care utilization
  • Morbidity and mortality measures
  • County economic activity
  • Socioeconomic and environmental variables
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
  • Provides current information about new data releases and publications
  • Subscribers can post messages to other members and exchange information
how to subscribe to hdas listserv
How to Subscribe to HDAS Listserv
  • In the body of an email message (leaving the subject line blank), type:
  • subscribe hdas-data your name
  • Send this message to:
nhds or nsas questions
NHDS or NSAS Questions?

Phone: 301-458-4321

Fax: 301-458-4032


new directions
New Directions
  • Beyond 20/20
  • Public use variance file
new directions123
New Directions
  • Clinical data
  • Evaluation of drugs in the NHDS
  • Two phase contract
    • Phase I – Research
    • Phase II – Field test