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Dear Shary, >1)- By changing the extent threshold (PET/SPECT/VBM) from zero to, >for instance 100, the set level p-value is affected because the >expected number of clusters and the actual number of clusters are >also affected. Am I correct? Yes, the probability of getting a cluster deceases with its size. >2)- Change of extent threshold does not affect corrected cluster >level p-value in my experience. Although it does change expected >number of clusters shown at the bottom of the SPM output. So, this >does not aid multiple comparison corrections as I test it on my data. That is right. The extent threshold simply filters out small clusters. Only the set-level p-value is affected by this. > 3)- This is strange: when I increase the extent threshold I get > smaller number of expected clusters which even becomes less than 1 > (changes from 2.44 to 0.37). Then, why the corrected cluster level > p is not smaller than uncorrected in this instance based on one of > your (or Will's) previous e-mails? Is it because the uncorrected > height threshold is not that high (<0.001) as it happens when I use > FWE (<0.000000000) where I get smaller corrected cluster level P > compared to uncorrected p? Still, the expected number of clusters > is less than one and should affect the corrected cluster level > p-value. Should it not? No. The corrected cluster level p is the chance probability of getting a cluster of the clusters size (or larger) in the search volume. This does not depend on the extent threshold. It depends on the expected number of clusters of any size and the uncorrected cluster p-value. Only the probability of getting the number of large clusters (or more) depends on the extent threshold. This threshold determines how large the cluster has to be. I hope this helps - Karl