Thanks Steve and Gwenaelle, I really appreciate your help! So with my new
design matrix below, what would the contrast matrices look like if I want to
1) find the effect of age on the differences in average FA
2) remove the effect of age
The ages of the controls were demeaned ONLY by the mean age of the controls.
The ages of patients were demeaned ONLY by the mean age of the patients.
Design matrix:
EV1(control) EV2(patient) EV3(ageControlDemeaned) EV4(agePatientDemeaned)
0 1 0 4
0 1 0 -2
...
...
1 0 3 0
1 0 -1 0
randomise -i all_FA_skeletonised -o tbss -m mean_FA_skeleton_mask -d
design.mat -t design.con -n 500 -c 3 -D -V
Thanks so much for your time and patience!
Cheers,
Russell
-----Original Message-----
From: FSL - FMRIB's Software Library [mailto:[log in to unmask]]On
Behalf Of Steve Smith
Sent: Thursday, July 17, 2008 7:14 AM
To: [log in to unmask]
Subject: Re: [FSL] age and age*group interaction for randomise and tbss
Hi,
The problem is that the design is rank deficient - at least one EV is
a linear combination of others. I would use the following design:
EV1(control) EV2(patient) EV3(demeanedagecontrol)
EV4(demeanedagepatient)
1
0
3 0
1
0
-3 0
0
1
0 4
0
1
0 -4
and e.g. a contrast [0 0 1 -1] asks for (one direction of) the age
interation.
Cheers.
On 16 Jul 2008, at 20:20, Russell Chu wrote:
> Hi,
> I processed some patients and controls with TBSS and am now trying
> to find
> the age effect and age*group interaction in randomise. Would someone
> please
> check my design and contrast matrices below? Also, in the design
> matrix, I
> subtracted the mean age from each individual subject's age; is this
> the
> proper way to demean the ages?
>
> Design matrix:
> EV1(control) EV2(patient) EV3(age) EV4(age*control) EV5(age*patient)
> 1 0 4 4 0
> 1 0 -4 -4 0
> 0 1 -9.8 0 -9.8
> ...
> ...
> 0 1 -1.2 0 -1.2
> 0 1 -3 0 -3
> 1 0 -4 -4 0
> 1 0 4.3 4.3 0
>
> Contrast matrix:
> EV1(control) EV2(patient) EV3(age) EV4(age*control) EV5(age*patient)
> control>meth 1 -1 0 0 0
> meth>control -1 1 0 0 0
>
> randomise -i all_FA_skeletonised -o tbss -m mean_FA_skeleton_mask -d
> design.mat -t design.con -n 100 -c 3 -V
>
> Also, after all permutations (only 100 for quick initial results) are
> finished, the following error occurs:
>
> terminate called after throwing an instance of
> 'NEWMAT::ConvergenceException'
> Aborted
>
> Any input on these issues would be greatly appreciated.
>
> Thanks,
> Russell
>
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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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