Thefirst level stratification and clustering within NASS/CDS was
accounted for using the“survey logistic”procedure available in SAS. Case
stratification in NASS/CDS is based on vehicle tow status, occupant
injury level, and hospitalization (NCSA, 2005). Thefirst level clusters are
represented by the primary sampling units (PSU's) located across the
United States. Each represents either a central city, a county surrounding
a central city, an individual county, or a continuous group of counties
(NCSA, 2005). A more detailed description of the NASS/CDS sampling
design methodology can be found in the Analytical User's Manual
(NCSA, 2005).