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: NAMCS samples visits to physician offices. NHAMCS samples visits to hospitals (outpatient departments, emergency departments, and ambulatory surgery locations).

Q: Why is NHAMCS sometimes referred to as three surveys?

A: 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 NAMCS and NHAMCS data?

A. The surveys were originally designed to provide national and regional (Northeast, Midwest, South, and West) estimates. There is also an item for metropolitan statistical area status (a yes/no field indicating whether the visit took place in a metropolitan or non-metropolitan area. From 2012-2015, the NAMCS sampling design changed to allow estimates to be made for the most populous states.  In 2012, the 34 most populous states were targeted, followed by 22 states in 2013, 18 in 2014, and 16 in 2015. Starting in 2016, NAMCS resumed an area-based sampling design which permits estimation for the nation, 4 Census regions, and 9 Census Divisions.  

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 both NAMCS and NHAMCS files from 1993-1999 were re-released to include them. We are hoping to eventually add masked sample design variables to NAMCS files from 1989-1992. 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 and how is it protected?

A. NAMCS and NHAMCS fall under Title 42, United States Code, Section 242K, which permits data collection for health research. We take your privacy very seriously. All information that relates to or describes identifiable characteristics of individuals, a practice, or an establishment will be used only for statistical purposes. NCHS staff, contractors, and agents will not disclose or release responses in identifiable form without the consent of the individual or establishment in accordance with section 308(d) of the Public Health Service Act (42USC 242m) and the Confidential Information Protection and Statistical Efficiency Act of 2002 (CIPSEA, Title 5 of Public Law 107-347). In accordance with CIPSEA, every NCHS employee, contractor, and agent has taken an oath and is subject to a jail term of up to five years, a fine of up to $250,000, or both if he or she willfully discloses ANY identifiable information about you. In addition, NCHS complies with the Federal Cybersecurity Enhancement Act of 2014. This law requires the federal government to protect federal computer networks by using computer security programs to identify cybersecurity risks like hacking, internet attacks, and other security weaknesses. If information sent through government networks triggers a cyber threat indicator, the information may be intercepted and reviewed for cyber threats by computer network experts working for, or on behalf, of the government.

The Cybersecurity Act of 2015 permits monitoring information systems for the purpose of protecting a network from hacking, denial of service attacks and other security vulnerabilities. 1 The software used for monitoring may scan information that is transiting, stored on, or processed by the system. If the information triggers a cyber threat indicator, the information may be intercepted and reviewed for cyber threats. The Cybersecurity Act specifies that the cyber threat indicator or defensive measure taken to remove the threat may be shared with others only after any information not directly related to a cybersecurity threat has been removed, including removal of personal information of a specific individual or information that identifies a specific individual. Monitoring under the Cybersecurity Act may be done by a system owner or another entity the system owner allows to monitor its network and operate defensive measures on its behalf.

1 “Monitor” means “to acquire, identify, or scan, or to possess, information that is stored on, processed by, or transiting an information system”; “information system” means “a discrete set of information resources organized for the collection, processing, maintenance, use, sharing, dissemination or disposition of information”; “cyber threat indicator” means “information that is necessary to describe or identify security vulnerabilities of an information system, enable the exploitation of a security vulnerability, or unauthorized remote access or use of an information system.”

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. NAMCS and NHAMCS meet both of these criteria. Additionally, disclosures may be made under a data use agreement with NCHS.

Q. Why is patient’s medical number collected?

A. NAMCS does not collect any personally identifiable data about patients such as patient’s name or address. However, we continue to collect the patient’s medical record number, which is considered directly identifiable information under the Privacy Rule. The medical record number is used by Census Bureau field representatives during the abstraction process to (1) collect patient information from multiple sites within the clinic, (2) identify the patient for correspondence with the office after the reporting period if the abstractor needs to update the patient information on any specific office visit, and (3) in the event that NCHS has to reabstract data for quality control. In general, the medical record number provides an opportunity for NCHS to check, confirm and verify the quality of the data the abstractor obtained during the abstraction process. If you are uncomfortable with providing the patient’s medical record number, simply request that this information be left blank on the computerized survey instrument.

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:

NCHS Confidentiality Office
Phone: 888-642-4159
Email: nchsconfidentiality@cdc.gov

Page last reviewed: January 30, 2017