Sorry.
The Puncorrected for voxel-level is not 0.068: it is 0.000.
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?
Thank you for your help.
>
>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.
>
>
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