Table 3 presents the prevalence rates of experimentation, repeated-use and SUDs for each substance at each age and wave of assessment for each gender. The prevalence of experimentation, repeated-use, and SUDs gradually increased between 12 and 22 years of age with few and inconsistent gender differences except in the case of alcohol and marijuana SUDs in young adulthood. The data also show a difference in the prevalence rates between 17- and 18-year olds tested at Wave 1 and Wave 2. The decreases from Wave 1 to Wave 2 may in part reflect the cohort effect observed in the MTF and other studies. An alternative explanation may be that our LTS sample (ascertained from birth records and followed annually from birth) which contributed many of the 17- and 18-year-old subjects at Wave 2, may have lower substance involvement than our CTS sample (ascertained as adolescents through the school system) that contributed the 17- and 18-year olds at Wave 1. Another possibility is that the computerized administration of the CIDI-SAM at Wave 2 may have led to slightly different prevalence estimates than
