Many previous studies identified a unidimensional structure of SUDs for individual substances. These findings are robust enough that a unidimensional structure of SUDs will be implemented in the DSM-5. [2] DSM-5 eliminates the abuse/dependence distinction and describes SUDs as a continuum, by including severity specifiers such as moderate versus severe, rather than just a categorical diagnosis. [3,4] Previous research strongly supports a unidimentional structure of DSM-IV SUD symptoms within specific substances, including alcohol [5–9], marijuana [8–11], and cocaine [9–11]. Non-DSM measures of problems with alcohol [12–16] and marijuana [17–18] also show evidence of unidimensionality. While many of these studies relied on self-report inventories, unidimensionality of substance use and problems has also been reported for a combination of self-report problem inventories with confirmation of use by biologic markers of substance use (e.g., saliva or urine samples) for marijuana, cocaine, and other illicit substances. [19] This wealth of prior research supports the unidimensionality of substance use and problems within individual substances. We turn now to considering models of multi-substance use, as well as how these models compare across different demographic groups and sampling schemes.
