Key Concepts about Deciding Which NHANES I Variables to Include by Consulting the Documentation

Contents of Documentation

Your analysis will require a subset of the variables available in NHANES I. To decide which variables are needed in your analysis, you need to review the survey documentation. The survey documentation contains information you will need to complete you analysis.

• The documentation describes the demographic information included in the file. Every component contains demographic information, but only the demographic data for those participants included in that component is present on a specific component file. You must read the survey documentation to determine which subsample participated in the component.
• The documentation also includes the information specific to that component.  The documentation will describe how data were collected and any limitations associated with the data.
• The documentation will describe the different samples used. Some components were limited to participants in the detailed subsample, whereas other components were on the entire sample, or limited to selected age groups.  Always check the sample size description to be sure you know which sample you have for that component.
• The documentation file does not include variable names. They are only available on the files for SAS Code or Formatted SAS Code.  However, you can identify the variable by its "tape location" which is available on all those files.

The above information, as well as the survey documentation for other components, is available in the NHANES I Data Files Documentation Codebook 1971-75 page . Within the data documentation files there are several main sections (refer to the documentation file Table of Contents).  Each documentation file contains the same introductory sections on survey description, variance estimation, and data preparation and procedures.  The documentation files also contain a tape description summary, and then a detailed tape description which includes the data file item description, codes, counts and the detailed notes that are specific to that data file (e.g. Biochemistry data). 

Task 1 focuses on the detailed Tape Description, codes, counts and notes section of the documentation. You may want to consult all of these to finish gathering background information on your variables.

The detailed Tape Description lists all the variables in the data file and includes the

• variable description,
• codes for all possible values or range of values,  
• counts for all possible values or range of values, and
• source of the data and a flag for important notes for that variable.

The frequency count for each variable item in the data file can be used to verify the sample size for a particular data item.  The notes section contains important information about a number of variables and should be consulted whenever a variable has a flag "see notes" or "notes".  These notes are located at the end of this documentation section and are organized in the same order as the variables appeared earlier in the section. The notes will tell you how some variables might be derived or recoded, important information about how the data was collected, and precise explanations of what information the variable encompasses.

Using the Documentation to Decide which Variables to Include

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 Medical Examination, Ages 1-74 years survey documentation file for "blood pressure." Some blood pressure measurements were taken by the physician on all the survey participants whereas other blood pressure measurements were taken by a nurse for the people included in the detailed subsample.  You will need to decide which blood pressure examination variables you would want to use in your analysis.

In another example, you may also want to include information on a medical history of blood pressure or hypertension, and the use of hypertensive medications.  Very similar information is available on both the Medical History Questionnaire Ages 12-74 Years and the Health Care Needs, General Medical History Supplement, and Respiratory and Cardiovascular Supplements, Ages 25-74 years. However, the Health Care Needs, General Medical History Supplement, and Respiratory and Cardiovascular Supplements, Ages 25-74 years component only includes persons in the detailed subsample, and if you choose to use data from this component instead of data from the Medical History Questionnaire Ages 12-74 Years, you will exclude the information on those topics for much of your sample. Unless, of course, your particular analysis is only focused on persons who were included in the detailed subsample.

You must read the documentation to obtain the pertinent details that will help you identify the correct variables suitable for your analysis.

IMPORTANT NOTE

Comparison between NHANES I data files and Continuous NHANES data files

The data files for NHANES I are structured differently than the data files for continuous NHANES. The continuous NHANES were first organized by survey cycle years, then by separate components for each topic area, plus separate links for the documentation, codebook, and frequency tables for each of the topic specific data components.

NHANES I is not structured around survey year.  There are also several key differences to keep in mind when you search and locate variables from NHANES I data:

1. There is no "variable list" file or link in NHANES I as in continuous NHANES.  You should check the Contents and the Tape Description Summary, and if indicated, the Detailed Notes to determine which of the files (there may be more than one) are likely to contain your variables of interest. For example, there are blood pressure history questions in both the Medical History Questionnaire, Ages 12-74 years and the Health Care Needs, General Medical History Supplement, and Respiratory and Cardiovascular Supplements, Ages 25-74 years, components, but the latter was only collected on the detailed subsample.
    
2. There are no separate links for codebook.  The "codebook" is embedded within the data file documentation and immediately follows the Tape Description Summary. It is not called a codebook.
    
3. There is also no separate link for frequencies. There are simple counts provided within the "codebook" (see above) next to the listing of each possible response to each question or measurement.

It is important to read the notes. This section immediately follows the "codebook". Variables which require further explanation or clarification will have the word or words see notes or see detailed notes written to their far right in the "codebook" section. It is important to follow through and read the pertinent text note in the notes section.