In addition to the Type II/B and Type I/A clusters, the cluster analysis used in the present study also generated a third cannabis dependence cluster. This third cluster was infrequent (n=18, 8.9% of the total sample) and differed from the Type I/A cluster only in having an age of onset of first cannabis intoxication of 19.4 years (as opposed to 13.9 years, p < 0.001) and a mean age at interview of 38.3 years (as opposed to 26.8 years, p < 0.001) (Table 2). We believe that the most parsimonious explanation for the third cluster is that it represents a temporal cohort effect, possibly marking a period of increased cannabis availability in the 1980s. Alternatively, or in addition, this small third cluster may represent individuals who had onset of cannabis use later in life due to other environmental variables.
