Ambulatory Health Care Data
Frequently Asked Questions (FAQ’s)
Q: What do the letters in NAMCS and NHAMCS stand for?
A: National Ambulatory Medical Care Survey; National Hospital Ambulatory Medical Care Survey.
Q: What is the difference between NAMCS and NHAMCS?
A: The NAMCS samples visits to physician offices. The NHAMCS samples visits to hospitals (outpatient departments and emergency departments).
Q: Why is the NHAMCS sometimes referred to as three surveys?
A: The NHAMCS is made up of three components: hospital outpatient departments (OPD), hospital emergency departments (ED), and hospital-based ambulatory surgery locations (ASL).
Q. How are the data used?
A. NAMCS and NHAMCS data are used to statistically describe the patients that utilize physician services and hospital outpatient and emergency department services, the conditions most often treated, and the diagnostic and therapeutic services rendered, including medications prescribed. The data are used by public health policy makers, health services researchers, medical schools, physician associations, epidemiologists, and the print and broadcast media to describe and understand the changes that occur in medical care requirements and practices. The data are disseminated in the form of public health reports, journal articles, and microdata files.
Q: Can the ambulatory medical care surveys be used to find out how many people have a certain diagnosis?
A: No. The ambulatory medical care surveys (NAMCS and NHAMCS) are not based on a sample of the population. They are based on a sample of visits rather than a sample of people. The data can be used to find out how many ambulatory care visits were made involving a certain diagnosis. To get an idea of utilization of ambulatory care in the population, the number of visits can be divided by the population of interest to get a rate of visits for a diagnosis of interest.
Q: Must one always use a single year or care setting when analyzing the ambulatory medical care data?
A. No. Survey years with the same patient record form (survey instrument) can be easily combined. Years where the same question of interest is asked can be combined. Within years, the three care settings can be combined because they have different sampling frames.
Q: Is it possible to obtain State-level estimates from the NAMCS and NHAMCS data?
A. The surveys are primarily designed to provide national estimates. Geographic region (Northeast, Midwest, South, and West) and metropolitan statistical area status (a yes/no field indicating whether the visit took place in an urban or rural area) are the only geographic designations on the files. The first stage of sampling is the selection of a group of PSUs (primary sampling units). These are geographic segments composed of counties, groups of counties, towns and townships or minor civil divisions, or metropolitan statistical areas. They may cross State lines and will not necessarily be selected in every State. In fact, the surveys are not designed to sample ambulatory care visits in every State, and meaningful estimates cannot be made on a State-level basis.
Q. How can variances be calculated for NAMCS and NHAMCS estimates?
A. Prior to calendar year 2002, NAMCS and NHAMCS public use files did not contain the sample design variables that are needed by sophisticated computer software like SUDAAN, which computes standard errors while taking the complex multi-stage sampling design into account. The design variables are confidential and have never been released to the general public. However, in 2002, a 5-year research project culminated in a plan to mask sample design variables so that they could be released without fear of disclosure of survey participants. The 2000 public use files were released with these variables, and files from 1993-1999 were re-released to include them. We are currently working to add masked sample design variables to NAMCS files from 1989-1992, and to NHAMCS files. Please see the survey documentation for more information about using computer software like SUDAAN to calculate standard errors.
Another method for calculating variances for NAMCS and NHAMCS estimates which does not require using SUDAAN or similar software is to use a generalized variance curve as described in the public-use file documentation. Use of this curve will produce approximate standard errors for estimates of visits and drug mentions. The variances that are produced in this way, using methods explained in the survey documentation, will, in general, be less precise compared with those produced using SUDAAN.
Users with additional data needs may contact the NCHS Research Data Center to conduct on-site research with restricted survey files. More information about the Research Data Center is available on the NCHS website.
Q. Under what authorization does NCHS collect this information?
A. The NAMCS and NHAMCS fall under Title 42, United States Code, Section 242K, which permits data collection for health research. The confidentiality of the data is protected by Title 42, United States Code, Section 242m(d). Information collected in the surveys is used only for statistical purposes. No information that could identify a person or establishment can be released to anyone - including the President, Congress, or any court, without the consent of the provider. The Census Bureau staff, who are collecting the data for NCHS, sign an affidavit making them subject to the Privacy Act, the Public Health Service Act and other laws that require the data be protected. The NCHS and Census Bureau maintain a perfect record in protecting the privacy of health care providers and patients.
Q. How does the HIPAA Privacy Rule affect a physician’s or hospital’s decision to participate?
A. The final Privacy Rule has been published as required by the Health Insurance Portability and Accountability Act of 1996 (HIPAA). Health care providers who transmit financial and administrative health information electronically must comply with the Rule as of April 14, 2003. The Privacy Rule permits physicians and hospitals to make disclosures of protected health information without patient authorization for public health purposes or for research that has been approved by an Institutional Review Board with a waiver of patient authorization. The NAMCS and NHAMCS meet both of these criteria. Additionally, disclosures may be made under a data use agreement with NCHS.
Q. Why does patient’s name appear on the form if NCHS does not collect it?
A. We ask that the survey respondent keep that part of the form above the perforation, which does contain the patient’s name. We collect the remaining information. We want the respondent to keep the patient’s name so that if we need to contact him or her regarding missing or conflicting information from the abstract form, then we can give the physician or hospital staff our unique ID number, which is also contained on the top portion of the form that is retained. This will allow the survey participant to locate the patient’s medical record without disclosing the name to us.
Q. Will anyone be able to identify a health care provider in the survey data?
A. No, we are legally bound to assure the confidentiality of all responses including anything that might result in a physician’s practice or hospital being identified. The data files that are released for research do not include any provider or patient identifying information.
Q. Is participation mandatory?
A. No, participation is completely voluntary.
Q. Then why should physicians and hospitals participate?
A. NAMCS participation is important because without one physician’s participation, neither that physician nor others similar to that physician are represented in the national description of office-based patient care. Physicians are randomly chosen to represent not only themselves, but thousands of other physicians in the same geographic region and medical specialty. The same justification applies to the hospitals that participate in the NHAMCS.
For specific questions about how NCHS protects the information physicians and hospitals provide, contact:
Confidentiality Officer, NCHS
3311 Toledo Road, Room 7116
Hyattsville, MD 20782
Telephone: (301) 458-4257
Ambulatory and Hospital Care Statistics Branch
National Center for Health Statistics
3311 Toledo Road
Hyattsville, MD 20782-2064
- (301) 458-4600
- Contact CDC–INFO