Which test(s) should I use for RCT, 2 groups (intervention vs. comparator) and two DVs measured at multiple points (pre, post, 3, 6, 12 months)?

Question

I am planning an RCT to evaluate the short- and long-term effectiveness of a CBT intervention. There will be two groups: intervention vs. active comparator and two outcome measures: anxiety scale and depression scale. Measures will be assessed pre-intervention (baseline), post-intervention, and at 3, 6, and 12 months. I want to analyse the between group differences (difference in anxiety and depression scores for initiative vs. comparator) AND The within group differences (difference in anxiety and depression scores for initiative group at the different time points.

Please help if you can!

Answer 1

For such data you should employ a repeated measures ANOVA analysis (RMANOVA), providing the clinic visit timeframes are consistent. RMANOVA can generate a significance test result (p-value) for the between-group test (B), the within-group test (W), and the interaction for the between and within (BW). Otherwise, if the timeframe is inconsistent regarding clinic visits, then GLM-Mixed, or GEE longitudinal regression analyses would be preferred.