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
<gr...@nmr.mgh.harvard.edu <mailto:gr...@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
<gr...@nmr.mgh.harvard.edu <mailto:gr...@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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