Introduction
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 Survey (NHANES) was 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 participate in a health examination, which is conducted in a mobile examination center (MEC). The MEC allows for 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.
In March 2020, the NHANES program suspended field operations due to the coronavirus disease 2019 (COVID-19) pandemic. As a result, data collection for the NHANES 2019-2020 cycle was not completed and the collected data are not nationally representative. Therefore, data collected from 2019 to March 2020 were combined with data from the NHANES 2017-2018 cycle to form a nationally representative sample of NHANES 2017-March 2020 pre-pandemic data. Please see the NHANES 2017-March 2020 pre-pandemic data page for detailed information on this combined sample. These combined data are only for survey components that were collected in both the 2017-2018 and 2019-2020 cycles. Data from survey components that were only collected in 2019-March 2020 will be made available as a convenience sample through the NCHS Research Data Center (RDC). Any analyses based solely on the 2019-March 2020 data would not be generalizable to the U.S. civilian non-institutionalized population.
Target Population
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 groups to increase the reliability and precision of estimates of health status indicators for these population subgroups.
The oversampled subgroups in the 2019-2020 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.
Survey Objectives
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;
- 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); and
- Collect and maintain a national probability sample of serum, plasma, and urine samples for potential future public health emergency use and surveillance; and maintain a national probability sample of DNA samples for potential future public health emergency use and surveillance.
Data Collection Procedures
NHANES uses a complex, multistage probability design to sample the civilian, noninstitutionalized population residing in the 50 states and Washington, 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 (neighborhoods) containing a cluster of households.
- Selection of specific households within segments.
- Selection of individuals within a household.
The sample for the NHANES 2019-2020 survey cycle was selected using a 4-year sample design for 2019-2022. Each 2-year data cycle from this design was intended to be nationally representative and included 30 PSUs each, for a total of 60 PSUs in the 4-year 2019-2022 sample. This design followed methods from previous NHANES sample designs. However, the 2019-2020 NHANES operations were suspended in mid-March 2020 due to the pandemic. At that time, data collection was completed in 18 of 30 locations or PSUs in the 2019-2020 sample. Data collection was canceled for the remaining 12 PSUs. From 2019 to March 2020, 10,855 persons were selected for NHANES from 18 different survey locations with an 85.3% screener response rate. Of those selected, 6,306 completed the interview and 5,596 were examined, resulting in a response rate of 49.6% for the interview and 44.0% for the examination (response rate = [(sample size) / (screener sample size)] × (screener response rate: 85.3%)).
Contents of the 2019-2020 NHANES questionnaires are available on the NHANES website.
Procedure manuals and protocols for each exam component in 2019-2020 can be found on the NHANES website as well.
Guidance for NHANES Data Users
We strongly encourage data users, prior to any data analysis, to 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. A detailed online tutorial is also available to guide data analyses.
Key points:
- The 2019-March 2020 sample is not nationally representative and unbiased estimates for the NHANES target population cannot be produced because calculation of survey weights for this partial cycle is not possible due to incomplete data collection.
- Survey components collected in both the 2017-2018 and 2019-2020 cycles were combined to form the NHANES 2017-March 2020 pre-pandemic data. Nationally representative estimates can be calculated using these data.
- Survey components that were only collected in 2019-March 2020 will be made available as a convenience sample through the NCHS Research Data Center (RDC). Any analyses based solely on the 2019-March 2020 data would not be generalizable to the U.S. civilian non-institutionalized population.