One limitation of IRT analyses con ducted to date on cannabis use disorders is the general unrepresentativeness of the research samples. The two exceptions to this rule are the study by Teesson and colleagues (2002) and the study by Lynskey and Agrawal (2007) using general population samples of adults age 18 and older. In another study, the sample consisted of adult male twins selected from the Virginia Twin Registry (Gillespie et al., 2007). The remaining two samples consisted of adolescent (Martin et al., 2006) and adult (Langenbucher et al., 2004) substance abuse inpatients and outpatients. The second limitation of these studies was the selection of the target sample of cannabis users to form the basis of the IRT analyses. In these studies, analyses were conducted among cannabis users narrowly defined as those individuals who reported using cannabis at least five times in the previous year (Teesson et al., 2002), at least once a month for six months (Martin et al., 2006) or who reported using cannabis at lest five times during their lifetime (Langenbucher et al., 2004). In the other
