Ah sorry for the low level of info. I am aiming to do a simple group
comparison, where I'm comparing thickness between controls and obese or
T2DM patients, regressing out the effect of age, sex and, hypertension. So
I created one thickness file with fwhm 10 containing all participants of
the 3 groups.
The contrast file looks like this:

1 0 -1 0 0 0 for controls vs T2DM
1 -1 0 0 0 0 for controls vs obese
0 1 -1 0 0 0 for obese vs T2DM, although there I do not expect something to
happen

And the GLM works fine (differences in the first and second contrast), but
I was wondering whether the fact that the groups are unbalanced might
influence the results in some way.

Thanks,

Eelco




2016-10-13 11:17 GMT-03:00 Bruce Fischl <fis...@nmr.mgh.harvard.edu>:

> Hi Eelco
>
> it depends on how you setup your GLM. Are you trying to regress out the
> effects of obesity in some way? If you give us more details I expect
> someone else can answer your question (Doug!)
>
>
> cheers
> Bruce
>
>
> On Thu, 13 Oct 2016, Eelco van Duinkerken wrote:
>
> Thanks for the quick reply!
>> So if I understand correctly, the power of say the controls vs. diabetes
>> (indeed it is type 2 diabetes) comparison is constrained by the sample
>> size
>> of the obese group?
>>
>> 2016-10-13 11:02 GMT-03:00 Bruce Fischl <fis...@nmr.mgh.harvard.edu>:
>>       Hi Eelco
>>
>>       it isn't really a question of whether our implementation is
>>       senstitive to
>>       this. It's that in general your power will be constrained by the
>>       size of
>>       the smaller group (I assume this is Type 2 diabetes by the way).
>>
>>       cheers
>>       Bruce
>>
>>
>>       On Thu, 13 Oct 2016, Eelco van Duinkerken wrote:
>>
>>       > Hi all,
>>       > I am using FS with data from 2 different studies that were
>>       acquired on the
>>       > same MRI-machine with the same T1 and FLAIR sequences.
>>       Unfortunately, the
>>       > group sizes are not very balanced, with 31 controls, 16 obese
>>       and 32
>>       > diabetes patients.
>>       >
>>       > Is the GLM for thickness used in FS very sensitive to this
>>       unequal group
>>       > size?
>>       >
>>       >
>>       > Thanks for the help,
>>       >
>>       > Eelco
>>       >
>>       >
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>> --
>> Eelco van Duinkerken, PhD
>> Pontifícia Universidade Católica | Department of Psychology |
>> R. Marquês de São Vicente 225 | Gávea, Rio de Janeiro - RJ | CEP
>> 22451-900 |
>> Brasil |
>> E-mail: e.vanduinker...@vumc.nl | Phone: +55-21-35271855 |
>> http://lattes.cnpq.br/7180895567820901
>> &
>> VU University Medical Center | Department of Medical Psychology |
>> De Boelelaan 1117 | 1081 HV | Amsterdam | The Netherlands
>>
>>
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-- 
Eelco van Duinkerken, PhD
Pontifícia Universidade Católica | Department of Psychology |
R. Marquês de São Vicente 225 | Gávea, Rio de Janeiro - RJ | CEP 22451-900
| Brasil |
E-mail: e.vanduinker...@vumc.nl | Phone: +55-21-35271855 |
http://lattes.cnpq.br/7180895567820901
&
VU University Medical Center | Department of Medical Psychology |
De Boelelaan 1117 | 1081 HV | Amsterdam | The Netherlands
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