The National Center for Health Statistics (NCHS), Division of Health and Nutrition Examination Surveys (DHANES), part of the Centers for Disease Control and Prevention (CDC), has conducted a series of health and nutrition surveys since the early 1960's. The National Health and Nutrition Examination Surveys (NHANES) were conducted on a periodic basis from 1971 to 1994. In 1999, NHANES became continuous. Every year, approximately 5,000 individuals of all ages are interviewed in their homes and complete the health examination component of the survey. The health examination is conducted in a mobile examination center (MEC); the MEC provides an ideal setting for the collection of high quality data in a standardized environment.
Details of the design and content of NHANES and the public use data files are available on the NHANES website.
The NHANES target population is the noninstitutionalized civilian resident population of the United States. The NHANES design has changed periodically to sample larger numbers of certain subgroups of particular public health interest to increase the reliability and precision of estimates of health status indicators for these population subgroups.
In 2011, a primary sample design change was implemented in NHANES to oversample non-Hispanic Asians, in addition to the ongoing oversampling of Hispanics, non-Hispanic blacks, older adults, and low income whites/others. Because the total sample size in any year is fixed due to operational constraints, sample sizes for Hispanic persons and non-low income white and other persons were decreased in order to increase the sample sizes for Asians.
The oversampled subgroups in the 2017-2018 survey cycle were:
- Hispanic persons;
- Non-Hispanic black persons;
- Non-Hispanic Asian persons;
- Non-Hispanic white and other† persons at or below 185 percent‡ of the Department of Health and Human Services (HHS) poverty guidelines; and
- Non-Hispanic white and other† persons aged 80 years and older.
† Other: Non-Hispanic persons reported races other than black, Asian, or white.
‡ The cut-point for low-income oversampling was changed from ≤ 130% to ≤ 185% of the HHS poverty guidelines, starting 2015.
The major objectives of NHANES are to:
- Estimate the number and percentage of persons in the U.S. population and in designated subgroups with selected diseases and risk factors;
- Monitor trends in the prevalence, awareness, treatment, and control of selected diseases;
- Monitor trends in risk behaviors and environmental exposures;
- Study the relationship between diet, nutrition, and health;
- Explore emerging public health issues and new technologies; and
- Provide baseline health characteristics that can be linked to mortality data from the National Death Index or other administrative records (e.g., enrollment and claims data from the Centers for Medicare & Medicaid Services).
Data Collection Procedures
NHANES uses a complex, multistage probability design to sample the civilian, noninstitutionalized population residing in the 50 states and D.C. Sample selection for NHANES followed these stages, in order:
- Selection of primary sampling units (PSUs), which are counties or small groups of contiguous counties.
- Selection of segments within PSUs that constitute a block or group of blocks containing a cluster of households.
- Selection of specific households within segments.
- Selection of individuals within a household.
In 2017-2018, 16,211 persons were selected for NHANES from 30 different survey locations. Of those selected, 9,254 completed the interview and 8,704 were examined.
To facilitate the oversampling of the Asian population, survey materials were translated into Mandarin Chinese, both traditional and simplified, Korean, and Vietnamese. Recorded and written translations were also posted on the NHANES participants' web page, which included a short video that explained what was involved for NHANES participants. In addition to the above-mentioned Asian languages, this video was also available in Amharic, French, Haitian Creole, Hindi, and Spanish.
Staff participated in cultural competency training to help them recognize and respect cultural differences. Local interpreters were hired when necessary and were provided with translated glossaries of terms, hand cards, and exam scripts to minimize errors in interpretation. In addition, a professional medical interpreter phone service was available.
Contents of the 2017-2018 NHANES questionnaires are available on the NHANES website.
Procedures and protocols for each exam component in 2017-2018 can be found on the NHANES website as well.
Guidance for NHANES Data Users
We strongly encourage data users, prior to any data analysis, read all relevant documentation. Specific data file documentation can be found via the link next to the respective data file on the NHANES website. Data users should also reference the NHANES Analytic Guidelines.
NHANES 2017-2018 survey design and demographic variables are included in the Demographics file. NHANES public use data files can be linked by using the survey participant identification number (variable name: SEQN). Merging information from multiple NHANES 2017-2018 data files using SEQN ensures that the appropriate information for each survey participant is linked correctly. All data files should be sorted by SEQN before merging.
The NHANES 2017-2018 data files do not have the same number of records. For example, there are different numbers of subjects in the Interview and Examination samples. Additionally, the number of records in each data file varies depending on gender and age profiles for the specific component. Confidential and administrative data are not released. Some variables have been recoded to protect the confidentiality of survey participants.
Similar to the 2011-2016 survey cycle, an additional race-ethnicity variable (RIDRETH3) is included in the NHANES 2017-2018 demographic dataset to reflect the change in the sample design. RIDRETH1 is still included in the demographic dataset so that comparisons of NHANES 2017-2018 with previous survey cycles (without the Asian oversample) is possible. Refer to the Demographic Documentation and Analytic Guidelines for further detail.
Asians and Mexican Americans comprise about 13 and 15 percent of the un-weighted examined sample, respectively, for NHANES 2017-2018. These smaller sample sizes means that there could be limited power to detect differences between subgroups. Therefore, more caution is needed when performing analyses of the NHANES 2017-2018 data for these population subgroups and four years (2015-2018) may be necessary for some analyses to be reliable. Please refer to the NHANES Analytic Guidelines for more detailed information.
Finally, a detailed online tutorial is available to guide data analyses.