Hey All,

I am sorry to jump into this discussion.  I know how to check the slope difference between groups. I want to ask help from you to confirm that if difference in thickness in some brain areas arising from (0 0 1 -1) happens, it means that there is a group * age interaction, right?

Then, the only option is DODS rather than DOSS. If no difference in thickness is found for this contrast, it means that there is no interaction. Then DOSS is more appropriate. Am I right?

Lickey


On Mon, Jan 13, 2014 at 12:03 AM, Douglas Greve <greve@nmr.mgh.harvard.edu> wrote:

If you have two groups and a single covariate, when you run it with mri_glmfit create a contrast with [0 0 1 -1]. In QDEC, it will be one of the default contrasts (something like "do the two groups differ in slope?")
doug



On 1/12/14 5:31 AM, amirhossein manzouri wrote:

Thanks Doug and how can check if the slopes are different?

On Jan 12, 2014 12:33 AM, "Douglas Greve" <greve@nmr.mgh.harvard.edu> wrote:

Use DODS to test whether the slopes differ between the three groups. If they do not, then use DOSS.
doug




On 1/11/14 5:03 PM, amirhossein manzouri wrote:

Thanks Doug for the prompt reply. I just want to if DODS is ok to be used in my case or I should use DOSS?

On Jan 11, 2014 10:56 PM, "Douglas Greve" <greve@nmr.mgh.harvard.edu> wrote:



Hi Amirhossein, don't use the DOSS feature in QDEC because there is a bug such that 
it does not always do the right thing. Sorry, I sent out an email 
about 8mo ago on this, but it is not easy to let people know about a bug 
once the bug is out there. If you want to use DOSS, use mri_glmfit directly. If you still have a question,

 then repost to the list.
 doug


On 1/11/14 1:22 PM, amirhossein manzouri wrote:
Hello all, 
I am using Qdec (version5.1) to study cortical thickness differences between groups having Age as nuisance covariate. I do not have assumption if the groups are different or not .  Using DODS and DOSS gives me completely different results. We have three groups including 41, 39, and 24 subjects in each.
My question is which results design matrix type is more reliable for such a study? And also what if I include a covariate of interest in my comparison?  
Thanks in advance,
Best regards,
Amirhossein Manzouri 

 


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