Hi,
gender needs to be demeaned. It is not necessary to split the age per group, unless you expect an interaction of age with group...
Cheers,
Gwenaelle
> De: Stijn Michielse <[log in to unmask]>
> Objet: [FSL] 3 groups randomise
> À: [log in to unmask]
> Date: Vendredi 11 février 2011, 14h57
> Dear FSL Experts,
>
> The project I'm working on has 258 subjects in the
> population divided over 3 groups. Processing in TBSS is
> straightforward and I have some questions regarding the
> randomise tool.
>
> Using the randomise tool, I first started creating the
> design matrix and contrast matrix (named design.mat and
> design.con). For performing a simple T-test everything is
> straightforward with contrasts 1 and -1 for corresponding
> groups. But things get complicated with the introduction of
> confounders. Our groups are not matched since we would like
> to include as many individuals as possible. Now we would
> like to add age, gender and handedness as a confounder in
> the model.
>
> Checking the JISCMail FSL Archives clue's regarding the
> demeaning of confounders pop up. Demeaning per group is
> necessary since our groups are not matched. Gender is a
> bi-directional (being either female or male) variable and
> doesn't need to get demeaned. In our case we have demeaned
> handedness since we apply an Oldfield scale (-100 is fully
> left-handed, +100 is fully right-handed, with value's in
> between). To know sure we do the right thing in analysing, I
> attached our design matrix and contrast matrix. In the
> design matrix the first column is group 1, second column is
> group 2 and the third column is group 3. As you might
> notice, row 48 has a group change since this individual is
> classified as patient after TBSS processing (some more
> changes are seen further on).
> For investigating the influence of confounders I added
> three extra columns; column 4 for age (demeanded per group),
> column 5 for gender (not demeaned) and column 6 for
> handedness (demeaned per group). Is it necessary to add
> specified columns per group for age, padding the other
> groups with 0? Later we may add more confounders if it
> survives.
>
> Executing the randomise tool with the two designs goes like
> this:
> randomise -i all_FA_skeletonised -o tbss -m
> mean_FA_skeleton_mask -d design.mat -t design.con -n 5000
> --T2 -V
>
> Can someone please review the attached design matrix and
> contrast matrix and give some advice?
>
>
> Kind regards,
>
> Stijn Michielse
> Research Assistant
> Dept. Psychiatry and Neuropsychology
> Maastricht University
> E-mail: [log in to unmask]
--------------------------------------------------------------------
Gwenaëlle Douaud, PhD
FMRIB Centre, University of Oxford
John Radcliffe Hospital, Headington OX3 9DU Oxford UK
Tel: +44 (0) 1865 222 523 Fax: +44 (0) 1865 222 717
www.fmrib.ox.ac.uk/~douaud
--------------------------------------------------------------------
|