I posted the below query last week but I have not yet had a response, as I havent been able to resolve this issue I am re-posting the query in the hope that someone (particularly relevant to Dr Gaser) might be able to help me.
I would be most grateful for any help.
Many thanks, Alexis
Hi all
I am using the VBM5 toolbox to examine group differences using a 2 sample t-test and I now wish to compute effect sizes for the differences.
I see below that Christian Gaser has provided the formula for how to do this, I am hoping that someone can help me by indicating which T statistic I should use for the calculation? As far as I can see the only t statistics given in the SPM T maps are for the individual voxels, however, I am running cluster level analyses and so wondered if there was a corresponding test statistic for the overall cluster that I could extract? The t statistics for the individual voxels are very high, resulting in very large effect sizes that are not consistent with what I would have expected from the literature.
I would be most grateful if someone could please tell me whether I am using the wrong T statistics?
Many thanks, Alexis
-----Original Message-----
From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]] On Behalf Of Christian Gaser
Sent: 03 May 2009 23:10
To: [log in to unmask]
Subject: Re: [SPM] How to caculate Cohen's d in T test in SPM
Hi Zude,
the calculation is quite easy:
2t
d = ----------------
sqrt(df)
Try the attached tool or use the imcalc function.
Best regards,
Christian
_______________________________________________________________________
Christian Gaser, Ph.D.
Assistant Professor of Computational Neuroscience Department of Psychiatry Friedrich-Schiller-University of Jena Jahnstrasse 3, D-07743 Jena, Germany
Tel: ++49-3641-934752 Fax: ++49-3641-934755
e-mail: [log in to unmask]
http://dbm.neuro.uni-jena.de
On Fri, 1 May 2009 18:23:14 +0800, zhu zhu <[log in to unmask]> wrote:
>Dear All,
>
>My reviewer asked me to report Cohen's d in a VBM analysis. It seems it
>is easy caculate effect size in ROI analysis, that I can caculate
>effect size in SPSS for ROI analysis. However, how can I caculate it in
>SPM contrast? If this is possible, is it the average effect size across
>a activation cluster will be reported in SPM? Thank you very much!
>
>Best regards,
>Zude.
>
