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Hi Monica,
That should be fine ( assuming “one test” means “separate model/design" )

Kind Regards,
Matthew
--------------------------------
Dr Matthew Webster
FMRIB Centre 
John Radcliffe Hospital
University of Oxford

On 30 Aug 2022, at 21:16, Keith, Monica <[log in to unmask]> wrote:

Sorry, I forgot to ask one more question:
3. For the second model (option2), can I add more covariates to evaluate the interaction with others or should I run one test per covariate?
 
Thanks again
 

From: FSL - FMRIB's Software Library <[log in to unmask]> on behalf of Monica Keith <[log in to unmask]>
Date: Tuesday, August 30, 2022 at 3:04 PM
To: [log in to unmask] <[log in to unmask]>
Subject: [FSL] Two groups with continuous covariate interaction

ATTENTION: This email originated from a sender outside of MCW. Use caution when clicking on links or opening attachments.
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Hi, I see in the FslWiki that there is a model for "Two group difference adjusted for covariate" (option1) and "two groups with continuous covariate interaction" (option2). I have a couple of questions regarding these two models.

1. My understanding is that the first one is used to test if FA (my dependent variable) is different between the two groups controlling for some covariate (i.e. age). The second one is used to test if the differences in FA between group1 and group2 vary with age. Is this correct?
2. The documentation says "if the interaction is significant, only the inferences of the interaction are of interest". Does this mean that if the second model gives significant voxels, I cannot test if there's differences between groups? So, I will never be able to say "FA is lower in patients than controls" because that difference is affected/depends on age?

Thanks

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