I have 30 patients and 30 controls and am interested in group differences in an MRI measure. This measure was taken in 5 different anatomical structures, and each structure exists bilaterally (there is a left and right structure). I am not interested in the difference in the measure between structures, but would like to know if there are side x group or side x structure x group interactions besides an overall group difference.
A similar paper used a three-factor repeated measures ANOVA: group as the between-subject factor, and then structure and side as within-subject factors. They then did post-hoc two-factor repeated measures ANOVAs to follow up. Another paper looking at just two structures simply did ANOVAs for each structure (group as between-subject, and side as within-subject).
I did the repeated measures analysis in SPSS and in the within-subjects effects and between-subjects effects tables, none of the effects or interactions are significant. However, the multivariate tests table indicates a tract x group interaction.
What exactly does this mean? I was told by another investigator that my significant result is from a MANOVA, and I can use it as long as subsequent analyses are also multivariate. Which post-hoc test would I use to find out which structure(s) are driving the interaction?
Thanks so much!