You need to demean age, which is where the negative numbers come from.
As the text for that example states:
"Age is mean centered in this model by subtracting the overall mean age
from each individual age. Importantly, this is done across all subjects
and not within group."
See the following for more on why we demean:
http://mumford.fmripower.org/mean_centering/
cheers,
-MH
--
Michael Harms, Ph.D.
-----------------------------------------------------------
Conte Center for the Neuroscience of Mental Disorders
Washington University School of Medicine
Department of Psychiatry, Box 8134
660 South Euclid Ave. Tel: 314-747-6173
St. Louis, MO 63110 Email: [log in to unmask]
On 9/18/14 2:30 PM, "Alshikho, Mohamad J." <[log in to unmask]>
wrote:
>Hi Michael,
>Do you mean something like this?
>MATRIX 1 (design.mat)
>>/NumWaves 2
>>/NumPoints 20
>>/PPheights 1 1
>>/Matrix>
>>1 0 39
>>1 0 48
>>1 0 38
>>1 0 50
>>1 0 43
>>1 0 27
>>1 0 34
>>1 0 30
>>1 0 41
>>1 0 41
>>0 1 23
>>0 1 39
>>0 1 42
>>0 1 44
>>0 1 31
>>0 1 42
>>0 1 33
>>0 1 31
>>0 1 52
>>0 1 37
>Actually I checked this
>example"http://fsl.fmrib.ox.ac.uk/fsl/fslwiki/GLM#Two-Group_Difference_Adj
>usted_for
>_Covariate"
>but what wondered me is why we have numbers with minus sign in the third
>column?
>
>Thanks a lot
>Mohamad
>________________________________________
>From: FSL - FMRIB's Software Library [[log in to unmask]] on behalf of
>Harms, Michael [[log in to unmask]]
>Sent: Thursday, September 18, 2014 3:18 PM
>To: [log in to unmask]
>Subject: Re: [FSL] Tbss_Covariate
>
>Hi,
>You need a single design matrix with demeaned age as your third column.
>See this example:
>http://fsl.fmrib.ox.ac.uk/fsl/fslwiki/GLM#Two-Group_Difference_Adjusted_fo
>r
>_Covariate
>
>cheers,
>-MH
>
>--
>Michael Harms, Ph.D.
>
>-----------------------------------------------------------
>Conte Center for the Neuroscience of Mental Disorders
>Washington University School of Medicine
>Department of Psychiatry, Box 8134
>660 South Euclid Ave. Tel: 314-747-6173
>St. Louis, MO 63110 Email: [log in to unmask]
>
>
>
>
>On 9/18/14 1:36 PM, "M. alshikho" <[log in to unmask]> wrote:
>
>>Dear FSL experts,
>>I am trying to add a covariate in my TBSS analysis. I wanted to eliminate
>>the confound of age covariate variable in my analysis (10 control, 10
>>patients). kindly, is the following correct:
>>
>>CONTRAST (design.con)
>>/NumWaves 2
>>/NumContrasts 2
>>/PPheights 1 1
>>/Matrix
>>1 -1
>>-1 1
>>
>>MATRIX 1 (design.mat)
>>/NumWaves 2
>>/NumPoints 22
>>/PPheights 1 1
>>/Matrix
>>1 0
>>1 0
>>1 0
>>1 0
>>1 0
>>1 0
>>1 0
>>1 0
>>1 0
>>1 0
>>0 1
>>0 1
>>0 1
>>0 1
>>0 1
>>0 1
>>0 1
>>0 1
>>0 1
>>0 1
>>
>>MATRIX 2 (design2.mat)
>>/NumWaves 1
>>/NumPoints 22
>>/PPheights 1 1
>>/Matrix
>>39
>>48
>>38
>>50
>>43
>>27
>>34
>>30
>>41
>>41
>>23
>>39
>>42
>>44
>>31
>>42
>>33
>>31
>>52
>>37
>>
>>
>>Then I used the following command line (adding the -x and the -D
>>options)
>>
>>randomise -i all_FA_skeletonised -o tbss -m mean_FA_skeleton_mask -d
>>design.mat -t design.con -x design2.mat -n 500 --T2 -V -D
>>
>>Thank you very much
>>Mohamad
>
>
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