Chapter 4. Description of Data Files

Structure of 2001 NHTS Pre-9/11 and Post-9/11 Data Files

The public use 2001 NHTS pre-9/11 and post-9/11 data are organized into four different data files, which are available to users in ASCII and SAS format. These four data files contain all information including weights, replicates and all other variables. (See Table 1.)

Derived Data Set

Beside these four data files, we derived two extra SAS data sets for each of these four data files.  One only contains ID variables, weights, and replicates.  Another contains weights and all other variables. The purpose of creating the extra data sets is user’s convenience in downloading manageable files – users can download just the data files needed for their analysis.

The Table 2 shows the data name, data file level, data file time frame, and the variables included for all of the 12 SAS data sets.

Data Dictionary

The file variables are identified by the variable name in the SAS versions. For each file variable, the codebook (Appendix B) contains:

* the variable type and length;
* whether the variable was identical to the one on the 1995 NPTS data set;
* the label, which is a brief description of the variable content;
* the section and item number of the questionnaire or other source of the data;
* value ranges and special codes;
* the unweighted frequency of responses for each value or code shown; and
* the weighted frequency of responses for each value or code shown.