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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!

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  • $\begingroup$ It's not clear to me exactly what "repeated measures analysis in SPSS" you ran. A univariate or multivariate model? It would be nice to present your model as well as the details of the results before any help could be offered. By "tract" in "tract x group" interaction do you mean structure? Also, how many subjects do you have? $\endgroup$ – bluepole Jun 16 '14 at 20:14
  • $\begingroup$ Apologies, I myself am not entirely sure what I did in SPSS--I only entered the one measure when defining factors for the repeated measures analysis (under the GLM options, I only see univariate, multivariate, and repeated measures). $\endgroup$ – moose Jun 17 '14 at 14:00
  • $\begingroup$ Also yes, tract = structure, and I have 30 patients and 30 controls. $\endgroup$ – moose Jun 17 '14 at 14:00
  • $\begingroup$ I've performed a similar analysis recently. Since this would most likely involve detailed discussions about the data structure, you may contact me directly (my ID at gmail) if you like. $\endgroup$ – bluepole Jun 17 '14 at 14:52
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You would need to run a simple effects analysis (one-way ANOVA) to determine where the significant difference comes from. So, first find significant interaction using MANOVA, then run simple effects analysis using ANOVA and report results.

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