Re: [Freesurfer] GLM in groups with unequal size

It will not

On 10/13/16 10:34 AM, Eelco van Duinkerken wrote:

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 <fischl@nmr.mgh.harvard.edu mailto:fischl@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
    <fischl@nmr.mgh.harvard.edu <mailto:fischl@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.vanduinkerken@vumc.nl
    <mailto:e.vanduinkerken@vumc.nl> | Phone: +55-21-35271855
    <tel:%2B55-21-35271855> |
    http://lattes.cnpq.br/7180895567820901
    <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.vanduinkerken@vumc.nl mailto:e.vanduinkerken@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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