The data documentation, codebook, and frequencies for each Demographic, Dietary, Exam, Lab, and Questionnaire component of the survey have been combined into a single master document, accessible from the single Docs link corresponding to that particular survey component.
This combined documentation file is one of several important references to use in analyzing the data. It should be used in conjunction with the specific Exam/Lab/Questionnaire procedures manual and/or questionnaire instrument, the analytic guidelines, and other information available on the NHANES website.
The data documentation section of the master document contains detailed information about each particular survey component including the component description, the eligible sample, protocol, and other analytic notes and references. The codebook section of the master document provides detailed information about each data item that is available in the public release data file. The first item listed in each codebook is the sequence number or 'SEQN'. This data field is used to merge different data files together and uniquely identifies each survey participant (sample person).
The following metadata (information about a data item) is incorporated into the NHANES codebooks:
In some questionnaire sections, 'check items' providing logic about item eligibility are included to better understand the flow thru a section. These 'check items' usually incorporate information about answers given in previous sections or question items. A user must also look at the item targets to understand who is eligible for a particular question. Please note however that the release data does not necessarily include all of the questions asked in a section. Data items may have been removed due to confidentiality, quality, or other considerations. Thus, it's possible that a dataset does not completely match all of the questions asked in a questionnaire section.
Each data file has been edited to include only those sample persons eligible for that particular section or component. A blank record for any sample person indicates that the person did not participate in that particular questionnaire section or examination/laboratory component, although they were eligible. Distinct responses of refused (7-fill) and don't know (9-fill) are included in the questionnaire data. Reasons for missing data in the exam and laboratory components can be found in the completion status and the comment codes for that section. Individual items for any section or component may be missing due to different targets within a section or component, or as a result of previous results or responses. To completely understand each section or component, the entire survey section/component content should be reviewed.