Dear Tom,
>For peak/magnitude statistics, I would disagree. Unless you know the
>exact voxel of the brain region of interest, you have a multiple
>comparisons problem on your hands: As you search over the volume of your
>apriori hypothesised regions, you are implicitly performing multiple
>tests.
>-Tom
I will try to qualify better the previous statement.
In the case of Katya the following should hold.
a)Having any prior information on the localization of the signal obviously
disqualifies the whole brain correction because the set of p-values is not
homogeneous any more.
In principle one should then apply the correction to the restricted
homogeneous set,e.g. the region. However, and here is the conundrum, we
should specify what we mean by prior information.
b)It is evident that Katia has seen a cluster under threshold in the PFC
that she believes more significant than others. She probably has some
evidence that distinguishes that cluster from different clusters within the
PFC. What Katia knows about the PFC probably implies the partition of the
p-values in the PFC in a number of subsets of homogeneous p-values. A
consistent approach should formalize Katia belief into a set of smaller
regions (probably many) within which the correction should be applied. Good
luck to her statistician.
c)Richard Perry mentioned a case where the region was selected because
activated in a previous study. Here I do not think the correction should be
applied at all because there is no global null hypothesis to disprove.
Scientiic inferences (not probabilistic) should be based on a comparison of
the two patterns, effetc size, anatomical considerations, circumstantial
information etc.
Best regards
Federico E. Turkheimer
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