A further issue related to the DSM -IV classification of cannabis use disorders is whether abuse and dependence criteria are categorical conditions or may be better represented by a dimensional approach. Are abuse and dependence unique, distinct disorders? Or do the phenomenological presentations better fit continuous measures (i.e. dimensions)? This issue parallels recent calls to add a dimensional component to all psychiatric diagnoses (Krueger 2004; Krueger et al. 2005), and recently, researchers have answered this call by using item response theory (IRT). Published findings using IRT analyses have found strong evidence supporting a single latent dimension underlying DSM-IV alcohol abuse and dependence criteria and items (Krueger et al., 2004; Langenbucher et al., 2004; Proud foot et al., 2006; Kahler and Strong, 2006; Saha et al., 2006; 2007), and the less extensive IRT literature for DSM-IV cannabis use disorders (Teesson et al. 2002; Langenbucher et al. 2004; Lynskey and Agrawal, 2007; Martin et al., 2006; Gillespie et al. 2007) similarly found better fits for a model specifying a unidimensional latent trait of cannabis use disorder severity. These studies also identified few
