This contrast will do it: [1 -1 0 0]
But I would first test [0 0 -1 1] to see if the slopes are different. If not, then use [1 -1 0] with DOSS instead of DODS

On 12/14/2021 5:07 PM, Laura Willers de Souza wrote:

        External Email - Use Caution        

Yes! Sorry for that.

And I'm resending my fsgd file!

Laura.

_______________________________________________________________________________________________________________
Hello FreeSurfer Developers,

I want to perform an analysis of the difference between two groups in cortical thickness, having cortisol levels as covariate.


This is my model: cortical thickness ~ pathology (CU/CI) + CSF cortisol + pathology*CSF cortisol

From the GLM DODS analysis I want to know if: 
  • the regression coefficient is significant only for the difference between groups, 
  • or only for cortisol, 
  • or for both, 
  • or if the coefficient of the intercept between group and cortisol is significant.
Is my FSGD file (attached) correct? What would the contrasts look like?

Thanks in advance!

Laura Willers de Souza 

Master Student in Biochemistry

Zimmer Lab - Federal University of Rio Grande do Sul 

- Porto Alegre/RS - Brazil

______________________________________________________________________________________________________________________


Answer by Douglas N. Greve:


Hello FreeSurfer Developers,
I want to perform an analysis of the difference between two groups in cortical thickness, having cortisol levels as covariate. This is my model: cortical thickness ~ pathology (CU/CI) + CSF cortisol + pathology*CSF cortisol 
From the GLM DODS analysis I want to know if:

  * the regression coefficient is significant only for the difference
    between groups,
this could possibly mean two things: the difference in offset [1 -1 0 0] or the difference in slopes ([0 0 1 -1] 
  * or only for cortisol,


[0 0 0.5 0.5] will test whether the cortisol slope is different than 0

  * or for both,


both what?

  * or if the coefficient of the intercept between group and cortisol
    is significant.


this does not make sense to me

Is my FSGD file (attached) correct? What would the contrasts look like?

                  
_________________________________________________________________________________________________

                  

                  
Answer by Douglas N. Greve:


                  

                  
The model is actually, two independent linear models:
thicknessCU = CUOffset + CUcortisol*CUslope
thicknessCI = CIOffset + CIcortisol*CIslope
There will be four regression parameters:
[CUOffset  CUslope CIOffset CIslope]



                  

                
              

            

          

        

      

      

      
De: Douglas
          N. Greve <dgreve@mgh.harvard.edu>

          Enviado: terça-feira, 14 de dezembro de 2021 18:54

          Para: Laura Willers de Souza
          <laura.w.desouza@hotmail.com>

          Cc: freesurfer@nmr.mgh.harvard.edu
          <freesurfer@nmr.mgh.harvard.edu>

          Assunto: Re: [Freesurfer] Questions about contrasts
        
 

      

      
Can you include the previous
        emails in your current email?

        

        
On 12/14/2021 3:46 PM, Laura
          Willers de Souza wrote:

        

        

          
        External Email - Use
              Caution        

          

            

              Dear Douglas,

            

               

            

              Thank you so much for your answer!

            

               

            

              Sorry, but I don't think I explained
                myself correctly in the previous email.

            

               

            

              I want to analyze the difference in
                cortical thickness between two groups (CU and CI),
                adjusting for a covariate (cortisol CSF) and the
                interaction between them.

            

               

            

              So my model would be: cortical thickness
                ~ diagnosis + CSF cortisol + diagnosis*CSF cortisol

            

               

            

              Using the GLM DODS, the contrasts,
                according to the wiki example (MailScanner
                      has detected a possible fraud attempt from
                      "secure-web.cisco.com" claiming to be https://surfer.nmr.mgh.harvard.edu/fswiki/Fsgdf2G1V), would be:

            

               

            

            

            
    
              
  • 
                To verify if only the diagnosis
                  regression coefficient is significant (diagnosis
                  changes cortical thickness regardless of cortisol):
    • 
              
  • 
                
    
                  1 -1 0 0
    
              
    • 
              
  • 
                
    
                  
    
                
    
              
    • 
              
  • 
                
    
                  To verify if the regression
                    coefficient of the interaction between diagnosis and
                    cortisol is significant (the two change cortical
                    thickness dependently):
    
              
    • 
              
  • 
                
    
                  0 0 1 -1
    
              
    • 
              
  • 
                
    
                  
    
                
    
              
    • 
              
  • 
                
    
                  To verify if only the cortisol
                    regression coefficient is significant (cortisol
                    changes cortical thickness regardless of diagnosis):
    
              
    • 
              
  • 
                
    
                  0 0 0.5 0.5
    
              
    • 
            

            

               

            

              Are these contrasts corret to represent
                the model we designed?

            

               

            

              

                Thank you so much
                  again!

              

                 

              

                Best regards,

              

                

                

              

                Laura Willers de Souza 

              

              

                

                    

                      Master Student in Biochemistry

                    

                      Zimmer Lab - Federal University
                        of Rio Grande do Sul 

                    

                      - Porto
                        Alegre/RS - Brazil

                  
                  

                    

                

              

              

                 

              

                

              

            

            

               

            

               

            

               

          

        

        

        
The information in this e-mail is intended only for the
          person to whom it is addressed. If you believe this e-mail was
          sent to you in error and the e-mail contains patient
          information, please contact the Mass General Brigham
          Compliance HelpLine at
          http://www.massgeneralbrigham.org/complianceline . If the
          e-mail was sent to you in error but does not contain patient
          information, please contact the sender and properly dispose of
          the e-mail.

        

        
Please note that this e-mail is not secure (encrypted). 
          If you do not wish to continue communication over unencrypted
          e-mail, please notify the sender of this message immediately. 
          Continuing to send or respond to e-mail after receiving this
          message means you understand and accept this risk and wish to
          continue to communicate over unencrypted e-mail. 
        

      

      

      

      
_______________________________________________
Freesurfer mailing list
Freesurfer@nmr.mgh.harvard.edu
https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer