Secondary evaluations of MTA 36-month outcomes: propensity score and growth mixture model analyses.

OBJECTIVE To evaluate two hypotheses: that self-selection bias contributed to lack of medication advantage at the 36-month assessment of the Multimodal Treatment Study of Children With ADHD (MTA) and that overall improvement over time obscured treatment effects in subgroups with different outcome trajectories. METHOD Propensity score analyses, using baseline characteristics and severity of attention-deficit/hyperactivity disorder symptoms at follow-up, established five subgroups (quintiles) based on tendency to take medication at the 36-month assessment. Growth mixture model (GMM) analyses were performed to identify subgroups (classes) with different patterns of outcome over time. RESULTS All five propensity subgroups showed initial advantage of medication that disappeared by the 36-month assessment. GMM analyses identified heterogeneity of trajectories over time and three classes: class 1 (34% of the MTA sample) with initial small improvement followed by gradual improvement that produced significant medication effects; class 2 (52%) with initial large improvement maintained for 3 years and overrepresentation of cases treated with the MTA Medication Algorithm; and class 3 (14%) with initial large improvement followed by deterioration. CONCLUSIONS We failed to confirm the self-selection hypothesis. We found suggestive evidence of residual but not current benefits of assigned medication in class 2 and small current benefits of actual treatment with medication in class 1.