The longitudinal nature of the data, coupled with the notable sample size (N=1386) derived from a community (non-clinical) twin sample, allows for these unique estimates of developmental trends in adolescent substance use independent of cohort effects. However, it must be noted that these estimates refer to the total diversity of substances used and do not take into account frequency or severity of use. Both licit and illicit substances were included; although the measure as a whole is internally consistent (α=0.72), there were notable differences in the proportions associated with use of the individual substances, likely due at least in part to differences in availability of licit and illicit substances. (Though it is important to note that only over-the-counter medications and inhalants can be legally acquired by participants in this sample, those substances which are widely legally available for older persons – i.e. alcohol and tobacco – may be more readily available to underage users than those which are illegal for all.)
