Dear all,
I have a (probably trivial) statistical question.
We performed a study on a rare genetic illness and found differences both in VBM analyses (using SPM software and adjusting for TIV) and TBSS.
The reviewer asked:
"Global grey matter volume, white matter volume and CSF volumes should be reported (these can be extracted from the VBM pre-processing procedure). If significant case-control differences are identified these should be accounted for in regional analysis. ie VBM and TBSS analyses may need to be discussed with and without co-variation for global differences."
I have no problem to calculate GM and WM volumes (probably CSF volume is more difficult, because often unreliable), but what is difficult to understand for me is why should I have to correct my statistics for global differences in GM/WM.
I will appreciate any help or reference. Thank you!
Angela
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