On Tue, 26 Aug 2008 11:57:46 -0400, Satoru Hayasaka <[log in to unmask]> wrote:
>> I am using SnPM to perform multiple-comparisons correction,
>> by calculating the p-val of getting the observed
>> maximum suprathreshold cluster size.
>> I would like to display this corrected p-val in the output figure.
>
>The SnPM online manual is somewhat out of date. In your SnPM results,
>you should look at the column under p_FWE-corr for corrected p-values
>for clusters.
Many thanks for the mail.
If I understand SnPM's maximum suprathreshold cluster-size
p-value correction correctly, then this observed maximum size
is meant to be a property of the whole brain, not of any individual cluster.
i.e. "Given a whole brain of voxels with permuted labels,
what is the prob that I would end up with any cluster anywhere
in the whole brain whose size Kmax is greater than the value k?"
The very fact that this gives us just one single p-value, for the whole brain,
is, as far as I understand it, the reason why this max-cluster-size trick
can be used for performing multiple comparisons correction.
There is only one comparison made, with one p-value.
The p_FWE-corr values listed in the results table that SnPM prints out
are p-values for each individual cluster, as opposed to being the
one single p-value, P(Kmax>=k), which tells me what the chance
is that a brain full of permuted voxels would have given me
a cluster as big as the biggest one that is actually observed.
Whatever the p_FWE-corr values given for each cluster might be,
they are not, as far as I understand it, the volume-wide P(Kmax>=k)
that I would like to display.
I might very well have misunderstood this, in which case please correct me!
Many thanks,
Raj
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