Similar to the duality in conceptualization of suicidal behavior, there are two different methods for examining different aspects of substance use that include assessing sequential stages of use between different drug classes, or assessing patterns of use within a specific drug class. The onset of first use of different substances has often been characterized as occurring in sequential categorical stages (Kandel, 1975; Labouvie and White, 2002), which correspond to stages of escalation through different classes of drugs. These categorical stages begin with the first use of beer and/or wine, and then progress through first uses of hard liquor and/or cigarettes, marijuana, and finally use of multiple illicit drugs. This approach is used to describe the initiation of substance use involvement as a progression through drug classes. However, adequate classification of adolescent substance use patterns also requires consideration of how substance use progresses within each drug class. From this latter perspective, substance use progression within a drug class is viewed as transitions from initiation, continuation, and maintenance/escalation of use, to cycles of regression, cessation, and relapse (Clayton, 1992). Furthermore, progression within
