Hello,
I'm using SPM8 r4290. I have a more basic, but related, question: I have been running a few t-tests and multiple regressions which show "no significant superthreshold clusters" when initially corrected using FWE p = 0.05. When "none" is chosen instead of "FWE" and p set to 0.001, under the clusters section of the results output, pFWE is significant (e.g., 0.018). What does this mean? Is this the p value for the cluster after FWE correction for all voxels? Is it different from first correcting for multiple comparisons/FWE and reporting only the clusters that survive this threshold? When not using FWE, I set p = 0.001 and re-run the output after getting the "expected number of voxels" and entering that value.
Are there any references for this topic specific to SPM?
Thank you for your help,
-Julie
Julie E. McEntee, M.A., C.C.R.P.
Image Processing Technologist
Center for Neuroscience & Regenerative Medicine, NIH/USUHS
The Henry M. Jackson Foundation for the Advancement of Military Medicine
NIH Clinical Center
B1N264B
301-451-1869
Department of Psychiatry- Neuroimaging
Johns Hopkins University School of Medicine
600 N. Wolfe St./ Phipps 300
Baltimore, MD 21287
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>Hi Marko,
>thanks a lot for the quick response!
>As I understand it, this method is more sensitive to detect larger clusters, but small clusters may >not be detected (Forman et al 1995), right? Can I use the method also if I do not know if I should >expect a small or big cluster?
>Can I choose any voxel-level threshold, e.g. p < 0.0001 to reduce false positives already in the >first step? Or are there rules how to choose the threshold?
>I'm afraid I do not really understand how CorrClusTh works - I was a bit suprised that the result >comes immediately without a number of iterations... Is there a link for further information or could >you explain it a bit? Thank you!!!
>Best wishes
>Janina
-------- Original-Nachricht --------
> Datum: Tue, 19 Jul 2011 09:53:22 +0200
> Von: Marko Wilke <[log in to unmask]>
> An: [log in to unmask]
> Betreff: Re: [SPM] CorrClusTh_Cluster-size_correction
- Hide quoted text -
> Hi Janina,
>
> > I would like to compare fMRI data of two groups (14+14) in a 2nd
> > level analysis in spm. Is it OK to do a cluster-size correction with
> > http://www.sph.umich.edu/~nichols/JG5/CorrClusTh.m by Nichols and
> > Wilke (or other tools compatible with spm) INSTEAD of FWE or FDR?
>
> Obviously, the complicated stuff in that script was conceived and
> written by Tom :) but in any case, the idea is to generate a cluster
> size that "ensures" (at the specified level of significance) that these
> clusters did not arise by chance. As you can see, you first have to
> specify a voxel-level threshold, which is then used to derive the
> cluster size. The question as to whether you can use this approach
> instead of FWE or FDR is not an easy one to answer as these approaches
> control different things: the "regular" FWE/FDR approaches control the
> error on the voxel level, while the script determines the cluster size
> that, in combination with a (likely less-stringent) voxel-level
> threshold, results in a cluster-size corrected result.
>
> > What does the value "Expected voxels per cluster" in the spm results
> > table mean (as this is compleatly different from the value
> > calculated with the tool).
>
> This refers to the estimated smoothness of the images that is needed to
> calculate the correction for multiple comparisons on the voxel level,
> based on the Gaussian random field approach as used by FWE (see Matthew
> Brett's excellent tutorial on this one, at
> http://imaging.mrc-cbu.cam.ac.uk/imaging/PrinciplesRandomFields).
>
> Hope this helps,
> Marko
>
> --
> ____________________________________________________
> PD Dr. med. Marko Wilke
> Facharzt für Kinder- und Jugendmedizin
> Leiter, Experimentelle Pädiatrische Neurobildgebung
> Universitäts-Kinderklinik
> Abt. III (Neuropädiatrie)
>
>
> Marko Wilke, MD, PhD
> Pediatrician
> Head, Experimental Pediatric Neuroimaging
> University Children's Hospital
> Dept. III (Pediatric Neurology)
>
>
> Hoppe-Seyler-Str. 1
> D - 72076 Tübingen, Germany
> Tel. +49 7071 29-83416
> Fax +49 7071 29-5473
> [log in to unmask]
>
> http://www.medizin.uni-tuebingen.de/kinder/epn
> ____________________________________________________
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