Dear Freesurfers,

Sorry if the question sounds too basic.

1. I am running an analysis on qdec with paired groups - normal and patients,
To control for IntraCranialVolume(ICV),I am adding it as an 'nuisance factor',Is it correct? 
The suggested mode to add ICV to the tabled was to demean the ICV.
But when I subtract the mean values from individual subject ICV values,I noticed that some of the subjects have a negative demeaned ICV value. 
My question is - How should I proceed to include the data,ignore the negative sign(because I thought a negative ICV makes no sense) and just keep the values or retain the numbers with the sign.
Please correct me if I am going wrong in any of the steps.
However,I ran two analysis - one with the sign retained and other with sign removed ,I noticed that both have very different results.

2. If I want to see the effect due to lithium within the patient group (Between patients with and without psychosis) how should I design the matrix in GUI.
I designed the following format
Fixed factor :- Diagnosis - Psychotic-pt
                                      Non-Psychotic-pt
Co-variate :- Lithium use :- 1 - lithium user
                                        0 - non user
Nuisance factor :- ICV
Is this design correct?
If not how should I construct it ?
Also what query would contain the result for anticipated question
My intention is to control for Diagnosis and then see effect/change due to lithium use.

Thank you very much



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