In the Data Analysis System (DAS), what is the difference between multi-year files and combined files?

The combined files available in the Data Analysis System (DAS) are combinations of single-year Public Use Files (PUFs). The files include every variable that has been included in at least two years, or that may have value for trend analysis. They include the variable YEAR, so DAS crosstabs can show trends over time. Some variables are not included in PUFs, and certain statistical methods are applied in order to protect respondent confidentiality.

The multi-year files in DAS are combinations of Restricted Use Files (RUFs). The files only include variables that are comparable across all of the years included in the files. They do not include a year variable in order to protect respondent confidentiality, and only averages across all of the years can be calculated. A much larger number of variables are included in the RUFs than in the PUFs, though some detailed geographic variables are not included. The multi-year RUFs do not include every case from the single-year RUFs.