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 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
Department of Psychiatry- Neuroimaging
Johns Hopkins University School of Medicine
600 N. Wolfe St./ Phipps 300
Baltimore, MD 21287
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>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!!!
-------- 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
> Hope this helps,
> PD Dr. med. Marko Wilke
> Facharzt für Kinder- und Jugendmedizin
> Leiter, Experimentelle Pädiatrische Neurobildgebung
> Abt. III (Neuropädiatrie)
> Marko Wilke, MD, PhD
> 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
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