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Hi Douglas,


Actually I have one more question.


I went to look at the contrast examples on the wiki and in the example that fits my analysis (Two Groups (One Factor/Two Levels), One Covariate) it says "DODS only", and you mentioned in the previous email that my analysis turned out to be DOSS, because the interaction is non-significant.

So my question is why on the wiki (MailScanner has detected a possible fraud attempt from "secure-web.cisco.com" claiming to be https://surfer.nmr.mgh.harvard.edu/fswiki/Fsgdf2G1V) it's like "DODS only"? 
My fear is that I have not explained my analysis correctly to you.


Best,

Laura Willers de Souza 

Master Student in Biochemistry

Zimmer Lab - Federal University of Rio Grande do Sul 

- Porto Alegre/RS - Brazil


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De: Douglas N. Greve <dgreve@mgh.harvard.edu>
Enviado: domingo, 02 de janeiro de 2022
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

 

Dear Douglas,

Again, thanks for the reply.

Please check if I understand correctly:

  * First I do the contrast [0 0 -1 1] to test if there is an
    interaction between diagnosis and cortisol; if there is, I do DOSS
    [1 -1 0], if there isnt’t, I continue with DODS.
  * Having no interaction, I do the contrast [1 -1 0 0] to check if
    only the diagnosis changes cortical thickness regardless of cortisol.

Did I interpret correctly?
You have it backwards. If there is no interaction,  use DOSS. If there is an interaction, consult a statistician. Fortunately, there is no interaction (whew!) 

And how do I check if cortisol changes cortical thickness regardless of diagnosis
 (assuming there is no interaction)?





[0 0 1] with DOSS





Thanks again!

Best,

*Laura Willers de Souza*









 



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

Enviado: quarta-feira, 15 de dezembro de 2021 

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

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

Assunto: Re: Questions about contrasts









Dear Douglas,


Again, thanks for the reply.

Please check if I understand correctly:

  *   First I do the contrast [0 0 -1 1] to test if there is an interaction 
between diagnosis and cortisol; if there is, I do DOSS [1 -1 0], if there 
isnt’t, I continue with DODS.
  *   Having no interaction, I do the contrast [1 -1 0 0] to check if only the 
diagnosis changes cortical thickness regardless of cortisol.

Did I interpret correctly?

And how do I check if cortisol changes cortical thickness regardless of 
diagnosis (assuming there is no interaction)?

Thanks again!

Best,
Laura Willers de Souza















---------------------------------------------------------------------------------------------------



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

Enviado: terça-feira, 14 de dezembro de 2021

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



 





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



 



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

Enviado: terça-feira, 14 de dezembro de 2021 17:30

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

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

Assunto: Re: Questions about contrasts



 



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



 



 



---------------------------------------------------------------------------------------------------



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

Enviado: terça-feira, 12 de dezembro de 2021 18:23

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



 





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]





 



 



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

Enviado: terça-feira, 12 de dezembro de 2021 10:48

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



 







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?







 



 



---------------------------------------------------------------------------------------------------



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

Enviado: terça-feira, 09 de dezembro de 2021 19:50

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

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

Assunto: Questions about contrasts



 



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