SPSS (version 11 for Mac, SPSS, Inc., Chicago, IL) was used to generate hierarchical average linkage and K means cluster analysis from clinical variables selected a priori as likely to contribute to the heritability of cannabis dependence. These variables were: age of first cannabis use, the number of childhood (<15 years) and adult (>=15 years) antisocial symptoms, the number of co-morbid lifetime anxiety and affective disorders, the number of DSM-III-R cannabis dependence criteria, the number of cannabis withdrawal symptoms, and severity of cannabis use. The number of cluster solutions was also specified a priori as two, three, and four. The number of cluster solutions was kept low in order to generate cluster solutions likely to be clinically meaningful.
