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. This survey is a major source of public health data, conducted by the Centers for Disease Control and Prevention’s (CDC) National Center for Health Statistics (NCHS). By combining interviews, physical examinations, and laboratory tests, NHANES provides a comprehensive picture used to monitor health trends, inform public health policy, and guide interventions across the nation.
Overview of NHANES Data Structure
Understanding the structure of NHANES data is foundational for analysis. The survey is continuous, but public-use data is released in two-year cycles (e.g., 2017–2018) to ensure adequate sample size and minimize disclosure risk. Since 1999, this continuous survey has resulted in the public release of over a thousand individual data files.
These files are categorized into four main components: Demographics, Questionnaire, Examination, and Laboratory. The Demographics file contains basic participant information (age, gender, income, race/ethnicity). Questionnaire files hold self-reported data on topics like diet and medical history. Examination files contain objective physical measurements (e.g., BMI, blood pressure), while Laboratory files provide results from blood and urine tests, covering biomarkers and environmental exposures. Researchers must download and link these component files separately, as they are not provided in one single dataset.
Step-by-Step Data Access and Download
The public-use data for NHANES is hosted directly on the NCHS website. Users navigate to the NHANES section and select the specific two-year survey cycle relevant to their research question. The website lists all data cycles, from the most recent to the earliest continuous surveys.
After selecting a cycle, users must review the extensive data documentation, which includes codebooks and analytic guidelines. This documentation is necessary for understanding variable definitions, survey procedures, and any changes that may have occurred between cycles. For example, a user interested in cholesterol levels would navigate to the Laboratory data section to find the specific file containing the HDL cholesterol variable.
Each data component page lists the available files, documentation, and the data itself, often in the SAS Transport File format (`.XPT`). Although provided in a format originally used by SAS, these files can be imported into other programs such as R or Stata using specialized functions. Users download the file by right-clicking the link and selecting “Save Link As.”
Preparing and Merging Datasets for Analysis
Raw downloaded NHANES files require preparation before statistical analysis. The fundamental step is merging the separate component files into a single dataset. This is accomplished using the unique participant identifier, the Respondent Sequence Number, consistently labeled as SEQN across all NHANES files.
The SEQN variable acts as the common link, allowing researchers to connect a participant’s demographic information to their laboratory results. Merging must be performed as a one-to-one match using this ID variable to ensure all variables for a single respondent are correctly aligned. Statistical software (R, Stata, or SPSS) is necessary to execute this merging and subsequent preparation steps.
The next critical preparation step involves applying the survey design variables, including the sample weights. NHANES uses a complex, multistage probability design; analyzing raw data without weights will produce inaccurate estimates. Sample weights (e.g., examination or dietary weights) account for differential probabilities of selection and nonresponse, ensuring nationally representative results. Researchers must also use the stratification variables and primary sampling units (PSUs) provided in the demographic file to correctly calculate variances and standard errors.