The TVEM SAS macro was used for all analyses (available for download at methodology.psu.edu). TVEM is a direct extension of multiple regression that allows regression coefficients (i.e., intercepts and slopes) to be estimated as non-parametric functions of continuous time or age. These functions are non-parametric because TVEM requires no constraints on the shapes of the intercept and slope functions across age. All models used “sandwich” error estimation to adjust for the clustering of observations within adolescents (Williams, 2000). All analyses involving the intervention were intent-to-treat analyses. As such, intervention condition assignment, as opposed to intervention participation, was used as the intervention variable in the current analyses (53% of adolescents in the analytic sample were randomly assigned to intervention).
