Hi,
just to add to Steve's response:
Demeaning refers to substracting the mean from the respective entries Xi, i.e. in your case the scores. It centers the data around zero and is derived from z-transforming values (where you would divide by the standard deviation in order to get sd units - however, that would not make a difference in the GLM or randomise).
Hope that clarifies a bit further and is not redundant.
Cheers-
Andreas
________________________________
Von: FSL - FMRIB's Software Library im Auftrag von Steve Smith
Gesendet: Mi 06.12.2006 04:33
An: [log in to unmask]
Betreff: Re: [FSL] GLM GUI setup
Hi,
On 5 Dec 2006, at 19:09, Silver, andy wrote:
> Thanks Steve. I think we'd want to contrast the behavioral
> correlation
> between the two groups. So, are you saying that in the 2nd GLM I
> should
> have 2 groups and 4 EVs, with EV1 and EV2 being exactly the same as I
> did in GLM 1 and then EV3 and EV4 would be the demeaned scores padded
> with zeros?
Yes, except that I'm also saying that for this EV3-EV4 contrasting
you should really move EVs 1 and 2 into the confound (-x) matrix,
otherwise the permutation testing won't be quite right.
> Also, I don't actually know how to demean scores? Is there
> an FSL command that I can use to do this or is it something I need
> to do
> with a different program? Thanks again.
I normally use matlab but it should be easy also in Excel or any
stats package.
Cheers, Steve.
> -Andy
>
> -----Original Message-----
> From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On
> Behalf Of Steve Smith
> Sent: Tuesday, December 05, 2006 1:47 PM
> To: [log in to unmask]
> Subject: Re: [FSL] GLM GUI setup
>
> Hi,
>
> On 5 Dec 2006, at 16:16, Andrew Silver wrote:
>
>> I am trying to figure out how to setup randomise to do a correlation
>> between FA values and a behavioral measure. Our setup has a patient
>> group and a control group. If I'm reading the other questions
>> correctly I believe I should do 2 GLM_GUIs, with the first making the
>> design.mat file and the design.con file, and the second making the
>> .mat file for the -x flag. First off, is this the correct setup?
>
> Yes.
>
>> When I made the first
>> GLM_GUI I had 2 groups and 2 EVs. For each person in Group 1 I had
>> EV1=1
>> and EV2=0, and for Group 2 I had EV1=0 and EV2=1. I set the contrast
>> to be 1 -1. Is this the correct setup for my first GLM?
>
> Yes.
>
>> As for the second
>> GLM, I am confused on how to set this up. Should I have both
>> groups in
>> it? How many EVs should I have? Do I have 2 groups with 1 EV and
>> have
>> that EV column filled with their scores on the behavioral test?
>> When I
>> tried that I was told that only 1 group could have non-zero EV
>> values. I
>> am also confused as to whether I should have actual behavioral
>> scores in
>> the EV column or if they should be demeaned. Any help would be much
>> appreciated. Thanks.
>
> Mostly this all depends on whether you want to test whether all
> subjects in both groups correlate with the behavioural measure on
> average (in which case use a single EV with demeaned behavioural
> scores for all subjects and put the group membership EVs into the -x
> confoudn matrix) or whether you want to contrast the behvaioural
> correlation between the two groups (in which case the confound matrix
> is the same as above, and the "real" model has two EVs, containing
> the different group's behavioural scores, each demeaned before
> padding with zeros; the contrast is then [1 -1] etc.).
>
> Cheers.
>
>> -Andy
>
>
> ----------------------------------------------------------------------
> --
>
> ---
> Stephen M. Smith, Professor of Biomedical Engineering
> Associate Director, Oxford University FMRIB Centre
>
> FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
> +44 (0) 1865 222726 (fax 222717)
> [log in to unmask] http://www.fmrib.ox.ac.uk/~steve
> ----------------------------------------------------------------------
> --
>
> ---
------------------------------------------------------------------------
---
Stephen M. Smith, Professor of Biomedical Engineering
Associate Director, Oxford University FMRIB Centre
FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
+44 (0) 1865 222726 (fax 222717)
[log in to unmask] http://www.fmrib.ox.ac.uk/~steve
------------------------------------------------------------------------
---
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