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

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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