On Tue, Jun 20, 2017 at 3:58 PM, Douglas Greve <greve@nmr.mgh.harvard.edu> wrote:

On 6/20/17 10:43 AM, marco mcsweeney wrote:

Dear Doug,

thank you.

I will go ahead with DOSS through the command line. I did find significant differences (FDR corrected) with DOSS in QDEC.

Hopefully they will still be present through the command line. It will be interesting to see if I get the same results.

Sorry but just one more quick question. I think I read on the list that demeaning the age (mean age of all participants - real age) is not

required with DOSS. Is this correct?

Correct.

Also, should DODS be used with the demean age covariate rather than the real age when looking

for differences in cortical thickness or LGI between 2 groups?

You should check whether there is an interaction between group and age with DODS (in which case demeaning does not matter). If there is none, then go to DOSS. If there is an interaction, then things get complicated in terms of the interpretation. There is no demeaning or not demeaning that will help this.
Thank you so much again.

Best wishes

Marco
On Tue, Jun 20, 2017 at 3:29 PM, Douglas Greve <greve@nmr.mgh.harvard.edu> wrote:
On 6/20/17 10:14 AM, marco mcsweeney wrote:

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?

I don't quite remember what was going wrong. It was only under some circumstances, but I can't remember what those were. The correct path forward is to use the mri_glmfit command line program for DOSS.

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.

That means you can use DOSS.

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?

There's nothing you have to do other than specify doss after the FSGD file (and create a new contrast matrix).

Or should age be used as a nuisance factor instead (0 in the design matrix)?

do you mean contrast matrix? if so, then just specify 0 for the nuisance colum(s)
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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