Dear Doug,

thank you so much for getting back to me on this.

I am using Freesurfer version 5.3 with QDEV version 1.4 which gives me both DODS and DOSS design matrix options. I am a little confused about DOSS in QDEC as I have read quite a few publications which have used DOSS in QDEC before. Can you please let me know what the issue with DOSS in QDEC is?

Also, I have looked at age-cortical thickness interaction using DODS in Qdec with gender and diagnosis as fixed factors, age as covariate (demean age as well) and eTIV as nuisance. I did not find any difference in age-cortical thickness interaction between the 2 groups.

Also, I am still a bit unsure about using age as covariate in DOSS through the command line. Is this appropriate given that DOSS assumes the same slope in both groups? Or should age be used as a nuisance factor instead (0 in the design matrix)?

Thank you so much again for your help with this.

Best wishes

Marco

On Tue, Jun 20, 2017 at 2:02 PM, Douglas Greve <greve@nmr.mgh.harvard.edu> wrote:

DOSS should not be used with QDEC as it may or may not do the right thing. Is it not disabled? What version are you using?

To answer your question, if you want to use DOSS and you're interested in the diff bet groups, then first test for an interaction between group and covariate. If that is not significant, then use DOSS

On 6/20/17 5:05 AM, marco mcsweeney wrote:
Dear Freesurfer experts,

I have a quick question regarding the appropriate design (DOSS or DODS) to be used with QDEC when comparing 2 groups for cortical thickness and LGI values.

So far I have used DODS first to look for an age-cortical thickness interaction in both groups. I used the real ages and also demeaned the age variable in my analysis. I entered age as a covariate and as a nuisance factor also. I have gender and diagnosis as discrete and eTIV as nuisance in both analysis. Neither was significant.

After this I used DOSS in QDEC to compare groups groups for both cortical thickness and LGI.

My questions relates to the use of age as a covariate in DOSS. Is it appropriate to enter age as covariate in DOSS given that DOSS assumes that both groups have the same slope? I have looked through the archives and online for an answer to this but I am still a bit confused.

Any help or advice on this would be greatly appreciated.

Thank you so much in advance.

Best wishes

Marco
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