Hi Steve, It looks good except for using the mean eTIV of the subgroup. Please use the global average or don’t multiply by anything at all. Multiplying by the global average has the cosmetic effect of giving you a volume that is the range of subiculum volumes (rather than fractions <<< 1), but won’t change the results of your analysis. If you normalize each group separately, the question you’re answering changes (even though in practice, I suspect there won’t be large differences between the eTIV of the groups, particularly if you correct for gender).
You can also correct for eTIV with regression, by the way. In that case, I would suggest demeaning the eTIV first (for the same cosmetic purposes). Cheers, /Eugenio Juan Eugenio Iglesias Senior research fellow CMIC (UCL), MGH (HMS) and CSAIL (MIT) http://www.jeiglesias.com From: <freesurfer-boun...@nmr.mgh.harvard.edu> on behalf of Steve Petersen <marfne...@gmail.com> Reply-To: Freesurfer support list <freesurfer@nmr.mgh.harvard.edu> Date: Wednesday, April 22, 2020 at 11:36 To: "freesurfer@nmr.mgh.harvard.edu" <freesurfer@nmr.mgh.harvard.edu> Subject: [Freesurfer] Correlation between Hippocampal subfields - behavioural test External Email - Use Caution Dear Freesurfer experts, I would like to perform a correlation between the volume of subiculum and some behavioural measures. Before making that correlation, is it necessary to correct the subiculum volume with the eTIV? I have thought of doing it by scaling the subiculum volume as follows: subic_vol = mean_eTIV_group * subic_vol / eTIV_subject Is correct?? mean_eTIV_group = eTIV average of the group to which the subject belongs. Thanks in advance. Best regards, Steve.
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