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Understanding and Using

NAMCS and NHAMCS Data

Part 1 – Survey Overview and SETS

Susan M. SchappertAmbulatory Care Statistics

BranchDivision of Health Care

Statistics

National Center for

Health Statistics

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Overview

• Background

• Data Uses

• Survey Methodology

• User Considerations

• How to Get the Data

• SETS Hands-On Training

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NAMCS and NHAMCS

• National Ambulatory Medical Care Survey (NAMCS)

– Visits to office-based physicians

• National Hospital Ambulatory Medical Care Survey (NHAMCS)

– Visits to hospital emergency and outpatient departments

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History of NAMCS

• Planning began in 1967

• Inaugurated in 1973

• Fielded 1973-1981, 1985, 1989-present

• Database covering more than 30 years

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Original NAMCS survey goals

• National statistics

• Professional education

• Health policy formulation

• Medical practice management

• Quality assurance

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History of NHAMCS

• Planning began in 1976

• Inaugurated December 1991

• Fielded annually

• 15th year of operation

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How are NAMCS and NHAMCS data used?

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Data users

• Universities and medical schools• Medical associations• Government agencies• Health services researchers• Broadcast and print media

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Antibiotic prescribing rates at physician office visits for children

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Rate per 1000 visits

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Ambulatory care visit rates for white and black females for selected diagnoses

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NAMCS and NHAMCS Methodology

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NAMCS Sample Design

• Three stage design– 112 PSUs– Physician practices within PSUs– Patient visits within practices

• One-week reporting period

• For 2004-- 3,000 doctors sampled;

data collected for 25,286 office visits

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Scope of the NAMCS

• Basic unit of sampling is the physician-patient visit

• In scope visits:– Must occur in physician’s office– Must be for medical purposes– Administrative visits not sampled– House calls, emails, phone calls not

sampled

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Scope of the NAMCS

• Physicians must be:– Classified by AMA or AOA as primarily

engaged in office-based patient care– nonfederally employed;– not in anesthesiology, radiology, or

pathology – 65 percent response rate in 2004

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In-Scope NAMCS Locations

• Freestanding clinic/urgicenter• Federally qualified health center• Neighborhood and mental health centers• Non-federal government clinic• Family planning clinic• Health maintenance organization• Faculty practice plan• Private solo or group practice

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Out-of-Scope NAMCS Locations

• Hospital ED’s and OPD’s

• Ambulatory surgicenter

• Institutional setting (schools, prisons)

• Industrial outpatient facility

• Federal Government operated clinic

• Laser vision surgery

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NAMCS Scope – How Much is Missed?

• What about non-office based physicians?• NAMCS excludes physicians whose main activity is

teaching, research, administration, hospital-based, or who are unclassified as to activity

• 1980 and 1995-97 Complement Surveys tried to estimate missed volume of visits otherwise in-scope for NAMCS

• Results indicated that NAMCS estimates underestimate all office-based care by about 11 percent.

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NHAMCS Sample Design

• Multistage probability design• First stage sample of 112 PSUs• Hospitals within PSUs• Clinics within OPDs, ESA within EDs• Patient visits within clinics, ESAs• 4-week reporting period• 464 hospitals sampled in 2004; 36,589

ED visits and 31,783 OPD visits

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Scope of the NHAMCS

• Basic unit of sampling is patient visit• Emergency and outpatient departments of

noninstitutional general and short-stay hospitals

• Not Federal, military, or Veterans Administration facilities

• Located in 50 states and D.C.

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NHAMCS Scope – How Much is Missed?

• OPD was intended to be parallel to the NAMCS in the hospital setting

• General medicine, surgery, pediatrics, ob/gyn, substance abuse, and “other” clinics are in-scope

• Ancillary services are out of scope

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Data Collection• Bureau of the Census is our field agent• Introductory letter sent 2-3 months in

advance of reporting period• Induction interview to train staff, obtain

data on practice or facility characteristics• Physician’s office/hospital staff is

responsible for completion of Patient Record forms; Census abstracts as a last resort

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Data Collection

• Patient Record Forms (PRFs)– Nearly identical for NAMCS and OPD– Some differences for ED– Redesigned once every 2 years– Copies in your documentation and at our

website

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Data Items

• Patient characteristics – Age, sex, race, ethnicity

• Visit characteristics– Source of payment, continuity of care,

reason for visit, diagnosis, treatment• Provider characteristics

– Physician specialty, hospital ownership…• Drug characteristics added in 1980

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Multiple Response Fields

• Up to 3 reasons for visit, causes of injury, physician diagnoses can be reported for each visit (no cause of injury on NAMCS and OPD starting in 2005)

• Up to 8 medications and each medication can have up to 3 therapeutic classes and up to 5 ingredients

• Multiple procedure codes for NAMCS and OPD

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Coding Systems Used

• Reason for Visit Classification (NCHS)

• ICD-9-CM for diagnoses, causes of injury and procedures

• Drug Classification System (NCHS)

• National Drug Code Directory– switching to Multum starting with 2005 data

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Drug Data in NAMCS/ NHAMCS

Respondents can list up to 8 medications (including Rx and OTC medications, immunizations, allergy shots, anesthetics, and dietary supplements) that were ordered, supplied, administered, or continued during the visit. Each entry is called a drug mention. Visits with one or more drug mentions are called drug visits.Respondents are asked to report trade names or generic names only (not dosage, administration, or regimen). Can’t link drugs with diagnosis.

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Drug Coding in NAMCS and NHAMCS

• Drug entries on the Patient Record form are coded twice, using two separate classifications, and yielding two separate types of information

• All entries are coded “as written” using the Drug Entry Coding List– All entries are also coded according to their

generic substance(s) using a separate classification of generic substance codes

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Drug Coding in NAMCS and NHAMCS (cont.)

• Drug entry codes and generic substance codes are independent of each other

• For example, there is a code for an entry of “acetaminophen” on the Patient Record form in the Drug Entry Classification and a separate code for acetaminophen in the Generic Classification.

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Drug Characteristics

• Generic Name (for single ingredient drugs)• Prescription Status – Rx or OTC – caveats apply• Composition Status – single or multiple

ingredient• Controlled Substance Status – DEA schedule• NDC Therapeutic Class (4-digit)• Up to 5 Ingredients (for multiple ingredient

drugs)

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Drugs as Ingredients

• Generic substance codes are used for both single-ingredient and combination drugs. For example, acetaminophen can occur both as a single-ingredient generic drug and as an ingredient in a combination product. The same code is used for both.

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Example of Drug Codes

If doctor writes “Tylenol #3” on PRF, it is coded as:

32920 in the Drug Entry field

51380 in the Generic Name field (combination product)

50005 (acetaminophen) and 70231 (codeine) in the Ingredients field

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• NAMCS or NHAMCS drug data can be analyzed – at the visit level (for example, the number

of visits at which a particular drug was prescribed)

– or at the medication level (for example, the number of “mentions” of a particular drug at ambulatory care visits

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Some User Considerations

• NAMCS/NHAMCS sample visits, not patients

• No estimates of incidence or prevalence• No state-level estimates• We do not sample by setting or by non-

physician providers– Note that, in 2006, we include a stratum of

CHCs, and non-physician providers are sampled within CHCs

• May capture different types of care for solo vs. group practice physicians

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NAMCS vs. NHAMCS

• Consider what types of settings are best for a particular analysis– Persons of color are more likely to visit OPDs

and EDs than physician offices– Persons in some age groups make

disproportionately larger shares of visits to EDs than offices and OPDs

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Sample Weight

• Each NAMCS record contains a single weight, which we call Patient Visit Weight

• Same is true for OPD records and ED records

• This weight is used for both visits and drug mentions

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Reliability of Estimates

• Estimates should be based on at least 30 sample records AND

• Estimates with a relative standard error (standard error divided by the estimate) greater than 30 percent are considered unreliable by NCHS standards

• Both conditions should be met to obtain reliable estimates

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Sampling Error

• NAMCS and NHAMCS are not simple random samples

• Clustering effects of visits within the physician’s practice, physician practices within PSUs, clinics within hospitals

• Must use some method to calculate standard errors for frequencies, percents, and rates

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Calculating Variance with NAMCS/NHAMCS Estimates

• Generalized Variance Curve (GVC). This is the least accurate method.

• NCHS Research Data Center for access to actual design variables.

• Masked design variables on our public use files from 1993-2004. Allows users to run SUDAAN and similar software to do more sophisticated analysis.

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Calculating Variance with NAMCS/NHAMCS Estimates, cont.• 1993-2002

– public use files contain multi-stage design variables for use with SUDAAN WOR design option

• 2002-forward – public use files contain ultimate cluster design

variables (single stage) for use with SUDAAN WR design option, SAS, Stata, SPSS, etc.

• To analyze data across these periods, need to create CSTRATM and CPSUM variables for years prior to 2003 using code available at our website

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2001

multi-stage design variables

2003

2002

ultimate

cluster variables only

multi-stage design variables

and ultimate cluster design variables

History of Design Variableson NAMCS/NHAMCS Public Use Files

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Ways to Improve Reliability of Estimates

• Combine NAMCS, ED and OPD data to produce ambulatory care visit estimates

• Combine multiple years of data

• Aggregate categories of interest into broader groups.

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Caveat on Counseling Services

• Diagnostic services are reflected accurately on medical records, but counseling services may not be (Stange, 1998, 2004)

• NAMCS and OPD data may underestimate the amount of health habit counseling that occurs if it is not included in the medical record

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Nonsampling Error

• Frame coverage

• Reporting and processing errors

• Biases due to survey and item nonresponse

• Incomplete responses

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Minimizing Nonsampling Error• Improve sample frame for better

coverage• Encourage uniform reporting and

eliminate ambiguities• Pretest survey items and procedures• Perform quality control procedures –

consistency and edit checks• Train Census field representatives

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How to Get the Data

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http://www.cdc.gov/nchs/namcs.htmhttp://www.cdc.gov/nchs/namcs.htm

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Public Use Micro-data Files

• Downloadable files• NAMCS, 1973-2004• NHAMCS, 1992-2004

• CD-ROMs• NAMCS, 1990-2003• NHAMCS, 1992-2003

• Tapes/cartridges (NTIS)• NAMCS, 1973-1997• NHAMCS, 1992-1997

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Enhanced Public Use Files

• SAS input statements, label statements, and format statements (1993-2004)

• SPSS and Stata code for 2002-2004

• Masked sample design variables– Allow use of SUDAAN, Stata, etc.– Available for 1993-2004

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NCHS Research Data Center

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Advantages of the Research Data Center

Gain access to information not available on public use files– Patient: ZIP code linked income, education,

etc.– Provider: physician sex and age, board

certification, teaching hospital– Geographic: FIPS state and county codes– Special files and data supplements

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Research Data Center – cont.

• Can merge with contextual variables (e.g., ARF, NHIS, Census, NHDS)– Health status level– HMO penetration– Physician and specialist supply– Medicaid reimbursement– Air quality– Percent in poverty

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Data Center rules

• Submit a proposal

• Cannot use data to identify patients or providers or geographic location of providers

• Cannot remove data files

• Fee – onsite / remote / file construction

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Research Data Center

• E-mail: rdca@cdc.gov

• Website: www.cdc.gov/nchs/r&d/rdc.htm

• Call (301) 458-4277

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Additional Information• Call us at (301) 458-4600

• Email me at SSchappert@cdc.gov

• Visit our website

• Join the ACLIST. It’s a moderated newsgroup for persons interested in NAMCS/NHAMCS. It currently consists of about 2,400 subscribers.