Your analysis will require a subset of the variables available in NHANES. To decide which variables are needed in your analysis, you need to review the survey documentation. The survey documentation for each component is slightly different. Survey documentation for:
These files, as well as the survey documentation for other components, are available on the Survey Questionnaires, Examination Components and Laboratory Components page for each two-year cycle of the survey. The survey documentation files are Adobe Acrobat PDF files. Use the search feature to speed up your search for variables relevant to your analysis. Read the documentation for each "hit" in your search results carefully though as not every result returned will be relevant to your analysis.
For example, assume you are preparing for an analysis using blood pressure variables, and search the examination survey documentation file for "blood pressure." Some blood pressure questions and measurements are used for safety exclusions in the Cardiovascular Fitness portion of the examination survey. However, the main collection of standardized blood pressure measurements is conducted on all eligible participants aged 8 years and older in the Physician's Exam section of the survey. These are the blood pressure examination variables you would want to use in your analysis.
In another example, assume you are also including cholesterol variables in your analysis and you search the laboratory survey documentation for "cholesterol." Again, you will have many results, but only some of them are applicable to your analysis. Notice that some of the results are in the Miscellaneous Laboratory Assays and others are in the Blood Lipids section. Reviewing both portions of the documentation, you still may not know which variables to use, so you will need to review additional documentation to determine which variable would be appropriate.
You must read the documentation to know this information and identify the correct variables for your analysis.