Hi - if the groups are age-matched then you don't necessarily need to
include the age regressors in order to correct for aging. However,
putting the age regressors in can still help reduce age-related cross-
subject variance and so that model with contrasts such as [1 -1 0 0]
does achieve what you want.
Cheers.
On 23 Jul 2008, at 22:26, Russell Chu wrote:
> Hi Steve,
> Thanks for your reply. The 2 groups have been age-matched and I want
> to
> adjust the FA values by removing differences between groups due to
> aging.
> Given the design matrix below, does the contrast matrix [1 -1 0 0]
> (for one
> direction) give group differences with, or without, adjusting for age?
> Thanks so much for your time and patience!
>
> design matrix:
> EV1(control) EV2(meth) EV3(ageControl_demeaned) EV4(ageMeth_demeaned)
> 0 1 0 4
> 0 1 0 2
> ...
> 1 0 -1 0
> 1 0 3 0
>
> best,
> Russell
>
>
> -----Original Message-----
> From: FSL - FMRIB's Software Library [mailto:[log in to unmask]]On
> Behalf Of Steve Smith
> Sent: Tuesday, July 22, 2008 6:07 AM
> To: [log in to unmask]
> Subject: Re: [FSL] age and age*group interaction for randomise and
> tbss
>
>
> Hi,
>
> On 18 Jul 2008, at 18:40, Russell Chu wrote:
>
>> 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
>
> Not sure what you mean - are you saying that the groups are not age-
> matched?
>
>> 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.
>
> Correct.
>
> Cheers
>
>>
>>
>> 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
>>>
>>
>>
>> ---------------------------------------------------------------------------
>> 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
> ---------------------------------------------------------------------------
>
---------------------------------------------------------------------------
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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