We have a randomised controlled trial, with two groups of the same size (drug and placebo). A variable was measured before and after substance administration. To assess a treatment effect, two approaches were used:
- a repeated measure ANOVA, with time as within-subjects factor and group as between-subjects factor.
- a ANCOVA, with pre-treatment variable values as a covariate, pos-treatment as dependent variable and group as fixed-effect.
These two approaches led to different results:
- RM-ANOVA shows a main effect of time (p<0.0001), an interaction effect (p=0.013) and no group effect.
- ANCOVA shows no effect for group (p=0.094), neither for the baseline measure (p=0.092).
Can I conclude that there is a treatment effect based on the interaction effect? Or, since the ANCOVA shows no effect, this interaction effect could be only due a difference in baseline scores? How to report this result?