Survey resources

Estimating counts in national population surveys

The estimated national counts of persons with asthma (National Health Interview Survey), number of office visits for asthma to private physicians’ offices (National Ambulatory Medical Care Survey), hospital outpatient departments and emergency rooms (National Hospital Ambulatory Medical Care Survey), and asthma hospitalizations (National Hospital Discharge Survey) are calculated by weighting the number of respondents or visits by the survey weights. For information about national health surveys, please see the survey descriptions below.

Behavioral Risk Factor Surveillance System (

The state-based Behavioral Risk Factor Surveillance System (BRFSS) is a cross-sectional telephone survey that state health departments conduct monthly over landline telephones and cellular telephones with a standardized questionnaire and technical and methodologic assistance from CDC. BRFSS is used to collect prevalence data among adult U.S. residents regarding their risk behaviors and preventive health practices that can affect their health status. For some survey modules, a responsible adult answers questions in proxy for a child in the family.

In some instances, states design samples within boundaries of sub-state geographic regions. States may determine that they would like to sample by county, public health district or other sub-state geography in order to make comparisons of geographic areas with their states. In order to conduct the BRFSS, states obtain samples of telephone numbers from CDC. States then review their sampling methodology with a state statistician and CDC to make sure data collection procedures are in place to follow the methodology. If any change in sampling methodology is considered, states consult with CDC before making changes. The BRFSS uses two samples: one for landline telephone respondents and one for cellular telephone respondents. Since landline telephones are often shared among persons living within a residence, household sampling is used in the landline sample. Household sampling requires interviewers to collect information on the number of adults living within a residence and then select randomly from all eligible adults. Cellular telephone respondents are weighted as single adult households.

The BRFSS questionnaire is comprised of an annual standard core, a biannual rotating core, optional modules, and state-added questions. The standard core is the portion of the questionnaire that is included each year and must be asked by all states. Each year, the core includes questions about emerging or “late-breaking” health issues. After one year, these questions are either discontinued or incorporated into the fixed core, rotating core, or optional modules. The rotating core portion of the questionnaire asked by all states on an every-other year basis. The optional modules are sets of standardized questions on various topics that each state may select and include in its questionnaire. Once selected, a module must be used in its entirety and asked of all eligible respondents. If an optional module is modified in any way (e.g., if a question is omitted), then the questions will be treated as state-added questions (see below). In addition, there are state-added questions. States are encouraged to gather data on additional topics related to their specific health priorities through the use of extra questions they choose to add to their questionnaire. All questions included in the BRFSS are cognitively tested prior to inclusion in the questionnaire.

Asthma Call-Back Survey (

The Asthma Call-back Survey (ACBS) is a product of CDC’s National Asthma Control Program. The ACBS is conducted approximately 2 weeks after the Behavioral Risk Factor Surveillance Survey (BRFSS). BRFSS adult respondents who report ever being diagnosed with asthma are eligible for the ACBS. If a state includes children in the BRFSS and the randomly selected child has ever been diagnosed with asthma, then the child is eligible for the ACBS. If both the selected child and the BRFSS adult in a household have asthma, then only one or the other is eligible for the ACBS.

Annual state-level data are available for some states, however, multiple years (currently five years) of combined data are needed to have stable estimates. State participation varies each year. From the parent survey (BRFSS), the ACBS inherits a complex sample design and multiple reporting areas. These factors complicate the analysis of the ACBS. Some states vary from both BRFSS and ACBS protocol. These variations should be considered prior to analysis of these data. Information on the BRFSS deviations can be found in the document titled Comparability of Data which can be accessed at: when selecting an individual survey year. Also, new weighting (since 2011) and dual mode data collection (landline and cell phone data) may be an obstacle for trend analyses.

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National Health Interview Survey (

The National Health Interview Survey (NHIS) is a cross-sectional household interview survey of the civilian noninstitutionalized population of the United States. Sampling and interviewing are continuous throughout each year. The sampling plan follows a multistage area probability design that permits the representative sampling of households. Traditionally, the sampling methodology for NHIS is redesigned about every 10 years to better measure the changing U.S. population and to meet new survey objectives. A new sample design was implemented in the 2006 survey. The fundamental structure of the 2006 design is very similar to the previous design for the 1995–2005 surveys.

Because the NHIS is conducted in a face-to-face interview format, the costs of interviewing a large simple random sample of households and non-institutional group quarters would be prohibitive; randomly sampled dwelling units would be too dispersed throughout the nation. To achieve sampling efficiency and to keep survey operations manageable, cost-effective, and timely, the NHIS survey planners used multistage sampling techniques to select the sample of dwelling units for the NHIS. These multistage methods partition the target population into several nested levels of strata and clusters. In order to increase the precision of estimates of the black, Hispanic, and Asian populations, the current NHIS sample design oversamples black persons, Hispanic persons, and Asian persons.

The current NHIS questionnaire, implemented in 1997, has two basic parts: a Basic Module or Core and one or more supplements that vary by year. The Core remains largely unchanged from year to year and allows for trend analysis and for data from more than 1 year to be pooled to increase the sample size for analytic purposes. The Core contains three components: the Family, the Sample Adult, and the Sample Child. The Family component collects information on everyone in the family. From each family in NHIS, one sample adult and for families with children under age 18 years, one sample child are randomly selected to participate in the Sample Adult and Sample Child questionnaires. For children, information is provided by a knowledgeable family member aged 18 years or over residing in the household. Because some health issues are different for children and adults, these two questionnaires differ, but both collect basic information on health status, use of health care services, health conditions, and health behaviors.

The NHIS has collected information on the race and Hispanic origin of its respondents, following guidelines set forth by the Office of Management and Budget (OMB) in a policy known as OMB Directive 15. The NHIS provides information about race/ethnicity data in its “Frequently Asked Questions” guide found here:

For national NHIS estimates of some asthma outcomes and management measures, NHIS asthma supplement data are available periodically (e.g., 2002, 2003, 2008, 2013) and future data availability depends on funding.

National Ambulatory Medical Care Survey (

The National Ambulatory Medical Care Survey (NAMCS) is an annual probability survey of nonfederal, office-based physicians who provide direct patient care in the 50 states and the District of Columbia, excluding radiologists, anesthesiologists, and pathologists. NAMCS collects data from a national sample of over 2,000 physicians each year who provide information on nearly 30,000 visits. Because persons with multiple visits during the year may be sampled more than once, NAMCS estimates are for visits, not persons.

NAMCS uses a multistage sample design procedure and sampling weights applied to each record to provide nationally representative estimates. Visits to private, nonhospital-based clinics, and health maintenance organizations (HMOs) were within the scope of the survey, but those that occurred in federally operated facilities and hospital-based outpatient departments were excluded. A sample of office-based physicians who reported that they were in direct patient care was taken from the master files of the American Medical Association and the American Osteopathic Association.

National Hospital Ambulatory Medical Care Survey (

The National Hospital Ambulatory Medical Care Survey (NHAMCS) is a national probability sample survey of in-person visits made in the United States to EDs and outpatient departments (OPDs) of nonfederal, short-stay hospitals (hospitals with an average stay of fewer than 30 days) and those whose specialty is general (medical or surgical) or children’s general. EDs that operate 24 hours a day are considered within the scope of the ED component; EDs that operate fewer than 24 hours are included in the OPD component of NHAMCS. The hospital sampling frame for 2006 consisted of hospitals listed in the 1991 Verispan Hospital Database (formerly known as the SMG Hospital Database), and updated using hospital data from Verispan, L.L.C. About 500 hospitals are included in the sample from which about 1,200 outpatient clinics and 400 ED departments are selected. Because persons with multiple visits during the year may be sampled more than once, estimates are for visits, not persons. NHAMCS uses a multistage probability sample and sampling weights applied to each record to produce nationally representative estimates.

National Hospital Care Survey/National Hospital Discharge Survey (

The National Hospital Care Survey was initiated in 2011 that integrates inpatient data formerly collected by the National Hospital Discharge Survey with the emergency department (ED), outpatient department (OPD), and ambulatory surgery center (ASC) data collected by the National Hospital Ambulatory Medical Care Survey. The integration of these two surveys along with the collection of personal identifiers (protected health information) will allow the linking of care provided to the same patient in the ED, OPD, ASC, and inpatient departments.

Through 2010, the NHDS collected data from a sample of inpatient records acquired from a national sample of hospitals. Because persons with multiple discharges during the year could have been sampled more than once, estimates are for discharges, not persons. Only hospitals with an average length of stay of fewer than 30 days for all patients, general hospitals, and children’s general hospitals were included in the survey. Federal, military, and Department of Veterans Affairs hospitals, as well as hospital units of institutions, such as prison hospitals, and hospitals with fewer than six beds staffed for patient use, were excluded. NHDS collected data from a sample of approximately 270,000 inpatient records acquired from a national sample of about 500 hospitals.

National Health and Nutrition Examination Survey (

The National Health and Nutrition Examination Survey (NHANES) is a program of studies designed to assess the health and nutritional status of adults and children in the United States. The survey is unique in that it combines interviews and physical examinations. NHANES is a major program of the National Center for Health Statistics (NCHS). NCHS is part of the Centers for Disease Control and Prevention (CDC) and has the responsibility for producing vital and health statistics for the Nation.

The NHANES program began in the early 1960s and has been conducted as a series of surveys focusing on different population groups or health topics. In 1999, the survey became a continuous program that has a changing focus on a variety of health and nutrition measurements to meet emerging needs. The survey examines a nationally representative sample of about 5,000 persons each year. These persons are located in counties across the country, 15 of which are visited each year.

The NHANES interview includes demographic, socioeconomic, dietary, and health-related questions. The examination component consists of medical, dental, and physiological measurements, as well as laboratory tests administered by highly trained medical personnel.

Findings from this survey will be used to determine the prevalence of major diseases and risk factors for diseases. Information will be used to assess nutritional status and its association with health promotion and disease prevention. NHANES findings are also the basis for national standards for such measurements as height, weight, and blood pressure. Data from this survey will be used in epidemiological studies and health sciences research, which help develop sound public health policy, direct and design health programs and services, and expand the health knowledge for the Nation.

National Vital Statistics System (

Data on deaths in the United States are based on information about underlying cause of death from all death certificates filed in the 50 states and the District of Columbia, and are processed by National Center for Health Statistics (NCHS). Mortality statistics are based on information coded by the states and provided to NCHS through the Vital Statistics Cooperative Program and from copies of original certificates received by NCHS from state registration offices.

Page last reviewed: July 14, 2016