Hi Rutger - I'm afraid in order to give clear answer about whether it is
possible to adapt the paired-t-test model for your case we need to see a
little more detail about the design - I'm not sure why you need 18+1 EVs -
can you have 1 EV for each subject's mean and then one or two EVs
modelling the contrasts (eg t-tests) of interest?
Wrt your final question - you may well be able to take contrasts at second
level instead of at first, depending on the detail, though if the
second-level design is complicated (as probably here!) then you are almost
certainly better off going back and adding the contrasts at first-level
(which is very easy given that you can add these into an existing FEAT
On Thu, 22 Jan 2004, Goekoop, R. wrote:
> Hi Mark,
> Yes, you're right. However I have specific reasons to create separate
> contrast EVs for each subject (i.e. EVx = [1 -1]). My design-matrix becomes
> singular when I use a single EV / designmatrix as you specified.
> In my design (n = 18, two or three repeated measures (inputs) for each
> subject, and 18 + 1 EVs (or more if I want to model covariates)), I may have
> too many specified regressors. However the problem might also be a bit more
> complicated (since FEAT did not produce a warning for rank-deficiency) and
> may lie in an adverse balance in DOF / number of inputs, the number of EVs
> that I have to specify, and the amount of information contained in the data.
> To circumvent such a complex problem (Too many unknowns? Too little
> information?), I tried the approach of creating a single contrast EV for
> each subject and to do away with the group mean EVs as described in my
> previous mail. This would give a small reduction in the number of EVs and
> perhaps solve the problem. A 3rd level analysis (again without group mean
> EVs) would ten give me the desired group average of that contrast.
> Would this be a correct approach, or is there another way in which I can
> prevent my matrix from becoming singular?
> Thanks again,
> Rutger Goekoop
> Another reason to use higher level single-subject (contrast) EVs is to
> combine lower level cope-images in a statistically meaningful way in case
> you forgot to specify a lower level cope for the relevant combination of
> PEs..... Although dirty, do you think this approach would be valid?
> -----Oorspronkelijk bericht-----
> Van: Mark Woolrich [mailto:[log in to unmask]]
> Verzonden: Thursday, 22 January, 2004 3:55 PM
> Aan: [log in to unmask]
> Onderwerp: Re: [FSL] Always include a higher level group mean EV?
> I'm assuming:
> input 1 is subject1 conditionA
> input 2 is subject1 conditionB
> input 3 is subject2 conditionA
> input 4 is subject2 conditionB etc.
> Your right in that you need a group mean EV for each subject. However, you
> need just one EV for the paired difference (EV1 = [1 -1 1 -1 1 -1]).
> See the Paired Two-Group Difference (Two-Sample Paired T-Test) example in
> the FEAT web pages for how to do this:
> Cheers, Mark.
> Mark Woolrich.
> Oxford University Centre for Functional MRI of the Brain (FMRIB),
> John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK.
> Work: +44-(0)-1865-222782, Mobile: +44-(0)-7808-727745
> On Thu, 22 Jan 2004, Goekoop, R. wrote:
> > Dear FSL-users,
> > I'd like to create a single contrast between two paired 1st level
> > cope-images using a 2nd-level analysis:
> > Input: Group: EV1 EV2 EV3
> > 1 1 1 0 0
> > 2 1 -1 0 0
> > 3 1 0 1 0
> > 4 1 0 -1 0
> > 5 1 0 0 1
> > 6 1 0 0 -1
> > ...
> > Since each EV only contains data from a single subject, would it still be
> > necessary to include group mean EVs, indicating which inputs belong
> > (i.e. are derived from the same subject, are paired), for example:
> > ...EV4 EV5 EV6...
> > 1 0 0
> > 1 0 0
> > 0 1 0
> > 0 1 0
> > 0 0 1
> > 0 0 1
> > ...
> > Or would it suffice just to specify EVs1-3 (see above),
> > Thanks a lot,
> > Rutger Goekoop
> > Drs. R. Goekoop, MD.
> > Department of Neurology
> > Vrije Universiteit Medical Centre
> > De Boelelaan 1117, P.O. Box 7057
> > 1007 MB Amsterdam, the Netherlands
> > Phone: 0031-20-4440316
> > E-mail: <mailto:[log in to unmask]>
Stephen M. Smith DPhil
Associate Director, FMRIB and Analysis Research Coordinator
Oxford University Centre for Functional MRI of the Brain
John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK
+44 (0) 1865 222726 (fax 222717)
[log in to unmask] http://www.fmrib.ox.ac.uk/~steve