Several limitations are noteworthy. First, a large part of the sample is below the age of risk for initiation and progression to heavier use for each substance. This limitation is perhaps of most consequence to the analysis of marijuana use, as relatively few individuals (8%) in this sample had progressed to heavier use. However, adolescence is a key period for the initiation of substance use and one aim of the study was to examine the influences on initiation and progression specifically within this age group. We employed statistical methods that were appropriate for this specific sample. Given that progression to clinical dependence on any of the substances studied could not be examined, this leaves open the question of the relationship between progression from heavier use to clinical dependence.
