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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 ------------------------------------------------------------------------ ---