design for 2005 is slightly different from that for 2002–2004. First, the 2005 NSDUH is the first survey in a coordinated five-year sampling design that spans 2005 to 2009, so unlike prior year surveys that have a 50-percent overlap in primary sampling units (PSUs) between successive survey years, there is no such overlap between the 2005 and 2004 NSDUH. Second, the PSUs in 2005 are census tracts as opposed to clusters of census blocks in the 2002–2004 surveys. The 2005 NSDUH also applied Small Area Estimation methodology that divided the states (the first level of stratification) into equally-sized state sampling regions (the second level of stratification). Nonetheless, the sampling designs in 2005 and 2002–2004 are sufficiently similar to allow pooling survey data across these four years. With the help of an incentive payment of $30, NSDUH maintained a fairly high response rate for computer-assisted interviews (CAIs)—79 percent in 2002, 77 percent in 2003 and 2004, and 76 percent in 2005. However, the present study was based on public use data, which did not include all the original respondents due to a subsampling step used in the disclosure protection procedures. The pooled data across the four surveys resulted in a total
