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