josephdviviano/epitome

epi-randomize and covariate interactions

colinhawco opened this issue · 1 comments

colinhawco commented

Sonja is running an interaction analysis on a covariate between groups. To do the interaction, the covariate is split into two columns, with zeros entered for the subjects in the other groups. For example:

1 0 -2 0
1 0 -1 0
1 0 -1 0
1 0 1 0
0 1 0 2
0 1 -0 1
0 1 0 1
0 1 0 1

Where column 1 if group1, column 2 is group2, columns three is the covariate in group1, and column four is the covariate in group two.

She is under the impression epi-randomise is currently mean-centering columns 3 and 4 separately. This is not correct. When analyzing an interaction of a covariate, the covariate should be mean-centered across ALL subjects, then split into two columns.

https://fsl.fmrib.ox.ac.uk/fsl/fslwiki/GLM#Two_Groups_with_continuous_covariate_interaction

I didn't check this myself, so maybe it is correct and Sonja misunderstood the code.