Dear Dr Gianelli,

>Sorry.
>The Puncorrected for voxel-level is not 0.068: it is 0.000.

I'm relieved to hear it!

>I have another question for you.
>In the voxel-level Puncorrected is less than Pcorrected, rightly for the whole
>brain correction.
>Why in the cluster-level Pcorrected is less than Puncorrected?

I think that the confusion comes from the words 'less than'.  As I 
understand it, at the voxel level, your P uncorrected is 0.000 and 
your P corrected is 0.192.  At the cluster level your P uncorrected 
is 0.068 and your P corrected is 0.078.  In both cases the correction 
increases the P value, i.e. it decreases the significance.

So your statement '... the cluster-level Pcorrected is less than 
Puncorrected ...' cannot possibly be correct (and isn't in this case, 
from the numbers that you have given).

However, you could perhaps say that the Pcorrected result is 'less 
significant than' the Puncorrected result.  (N.b. this is slightly 
misleading, in that it is not really the result that has changed, but 
the type of significance that is being calculated.  But I think that 
you probably see what I mean).  The reason why you are applying a 
correction (in the case where you don't have an a priori hypothesis 
about this particular voxel) is that you may have overestimated the 
significance (or, more strictly, underestimated the probability that 
your null hypothesis might be correct), so you want your correction 
to reduce the level of significance, i.e. increase the P value.

>Thank you for your help.

You are welcome.  I hope that it is clearer now.

By the way, I like the alcoholic e mail address.

Best wishes,

Richard.


>Dear Dr Gianelli,
>  >
>>>I run SPM to analyze a fMRI-time series.
>>>I have a cluster in the Z-map with the result:
>>>
>>>cluster-level
>>>Pcorrected=0.078
>>>Ke=6
>>>Puncorrected=0.068
>>>
>>>voxel-level
>>>Pcorrected=0.192
>>>T=5.46
>>>Z=4.31
>>>Puncorrected=0.068
>>
>>Have you used small volume correction?  I only ask, because these
>>voxel-level results don't appear to have been corrected for the whole
>>brain.  Very little correction has been applied, suggesting that the
>>total number of resels corrected for is very low, whereas for a
>>whole-brain acquisition you might expect there to be hundreds of
>>resels.  The other possibility is that your data is very smooth.
>>
>>>If I see the voxel-level results I think that the activation is a
>>>false positive
>>>(Pcorrected=0.192).
>>
>>There is no reason to think that the result is a 'false positive'.
>>It simply hasn't reached the arbitrary level of 5% corrected
>>significance, and as a result you would not report it as a
>>significant deviation of the data from the null hypothesis (= no
>>activation).  However, it may well be a real effect that you just
>>didn't have enough statistical power to demonstrate conclusively.
>>
>>>If I see the cluster level I am puzzled: Pcorrected=0.078.
>>>My activation is a false positive or a true activation?
>>>In Z maps results is more meaningful the voxel-level or cluster-level?
>>>What's the difference between cluster-level and voxel-level?
>>
>>The voxel level and the cluster level statistics are asking different
>>questions.  The voxel level statistics are to do with the height of
>>the effect within a single voxel.  This question is asked
>>independently of the other voxels (except that all of the data was
>>smoothed before the stats was done, of course).
>>
>>The cluster level statistics is to do with the probability of
>>getting, by chance, a cluster of a given size or larger, when the
>>data is thresholded at the particular level that you have set.
>>
>>One is no more meaningful than the other.  If you had a large diffuse
>>activation, it is possible to imagine that you would have a
>>significant result at the cluster level, with lots of voxels just
>>managing to exceed the threshold, but a non-significant result at the
>>voxel level, because none of these individual voxels exceeds the
>>level defined as 'significant'.
>  >
>>On the other hand, we more usually find ourselves in the situation of
>>a relatively small cluster with highly significant results at the
>  >voxel level.  The cluster level may in this example not even come
>>close to significance, because this only depends on the number of
>>voxels exceeding your threshold which exceed you threshold, not on
>>the amount by which that threshold is exceeded.
>>
>>>Thank you in advance for your help.
>>
>>You're welcome.  You might also want to look at the following 
>>archived e mail:
>>http://www.mailbase.ac.uk/lists/spm/2000-04/0042.html
>>
>>Best wishes,
>>
>>Richard Perry.
>>
>>

-- 
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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