Dear Takanori,
> I am interested in the GRF and statisitcs in the spm. I carried out
> some simulations to understand thresholding processes in spm and had
> questions about spatial extent test.
>
> And I plot the theoretical probability of the spatial extent test when
> simulated images is thresholded at different Z values and different
> areas n.
>
> ......
> for u=1:500;for k=1:100;pnumax(u,k)= spm_P(1,[8 8],u*0.01,k,128*128);end;end
> ......
>
> From the graph,the uncorrected p-value is greater than the corrected p-value
> by the combination of a certain u and k.
> Why is this?
I think one explanation for this anomaly is that you have used very low
thresholds. This violates the assumptions of GFT. In our simulations
we generally use thresholds of about Z = 2.4 or more. Your thresholds
go down to 0.01. GFT results are only assymtopically correct at high
thresholds.
> In this simulation, the probability of getting one or more clusters
> of size k or more, above high threshold u, almost corresponds to
> the corrected p-value.
This probability is the corrected p value.
With very best wishes - Karl
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