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Subject:

Re: Cluster level - someone else will need to answer this one!

From:

Richard Perry <[log in to unmask]>

Reply-To:

Richard Perry <[log in to unmask]>

Date:

Mon, 9 Oct 2000 10:59:04 +0100

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (79 lines)

Dear Dr Giannelli,

I must admit that, in the cluster level statistics, I am not 
absolutely sure what the correction means (I stick to the voxel level 
myself).

I assume that you must be correcting for the expected number of 
clusters.  In your case, your threshold is so high (p < 0.05 
uncorrected) that your expected number of clusters comes out at 0.05.

This probably explains your odd results.  Normally, you might have 
expected to look at the statistics of, say, 30 clusters (on the basis 
of the null hypothesis) and therefore you would correct your p value 
upwards (in the direction of less significance) to correct for about 
30 independent comparisons (perhaps a bit less).  In your case, 
though, you have in fact corrected for the fact that you have made 
'fewer than one independent comparisons' and therefore corrected your 
significance downwards (in the direction of more significance).  If 
this is the case, then I suspect that you should ignore the corrected 
statistics, as the fact is that you must have made at least one 
observation!

If this is the explanation, then it should only be a problem when you 
set your spm threshold very high, and under these circumstances every 
cluster is significant even at the voxel level, so you don't really 
need the cluster-level statistics.  My guess is that if the expected 
number of clusters is less than one, then even without an a priori 
hypothesis, you can use uncorrected cluster-level statistics, and you 
should ignore the spurious 'corrected' values that spm has produced.

However, this is just guesswork, and someone with more experience 
will need to answer this one.  Sorry.

Richard.

>Content-Type: TEXT/plain; charset=us-ascii
>Content-MD5: wBw+0Ou9tSFSFVo4gj6p8w==
>
>Sorry, but in previous message I am not clear.
>
>In previous Z map I have in cluster-level:
>
>Pcorrected=0.078
>Ke=6
>Puncorrected=0.068
>
>and Puncorrected is rightly less than Pcorrected (0.068 < 0.078).
>
>Nevertheless I have some Z maps where, in the cluster-level, 
>Pcorrected is less than
>Puncorrected (Pcorrected < Puncorrected): I don't understand.
>
>I put an example in attachment.
>
>
>Thank you for your help again.
>
>
>
>
>
>
>
>Content-Type: APPLICATION/postscript; name="file1.ps"; x-unix-mode=0660
>Content-Description: file1.ps
>Content-MD5: 6BBqtBzEKjSlboFHgs7mlA==
>
>Attachment converted: Ravel:file1.ps (TEXT/mlpr) (000634A5)

-- 
from: Dr Richard Perry,
Clinical Lecturer, Wellcome Department of Cognitive Neurology, 
Institute of Neurology, Darwin Building, University College London, 
Gower Street, London WC1E 6BT.
Tel: 0207 679 2187;  e mail: [log in to unmask]


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