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SPM uses the random field theory based family wise error correction to calculate the corrected p. Basically, it first estimates the smoothness of the data and then estimate the resel size. Instead of using # of voxels, it uses # of resels to calculate the corrected p value. --------------------------------------------- Ze Wang, Ph. D Center for functional neuroimaging, Dept of Neurology, Medicine school, University of Pennsylvania 3400 Spruce Stree, 3W Gates building, Philadelphia, PA 19104 Tel: 215-662-7341 -----Original Message----- From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]] On Behalf Of Hong Gu Sent: Tuesday, February 19, 2008 2:16 PM To: [log in to unmask] Subject: Re: [SPM] Combine spatial extent and peak intensity to test for activation >The cluster-extent correction typically works in two steps: first >applying a height threshold (also known as the primary threshold, or >cluster-defining threshold), then second calculating corrected p-values >for the cluster extents formed at that height threshold. In the example >you presented, I imagine the primary threshold was p<0.01 (uncorrected, >based on T-map intensity) and the clusters were selected for having >corrected p-values p<0.05. Needless to say, these results probably >weren't generated by the combined extent-peak test of Poline et al. Uncorrected at p<0.01 and corrected at p<0.05 is a possible explanation. But in another map of this same paper, they also used a height and extent thresholds at p<0.001. In another paper, the author even used a joint height and extent threshold of p<0.0001. I think it's rare to use corrected p value at 0.0001, isn't it? >SPM by default produces and displays corrected and uncorrected p-values >for clusters and local maxima when you assess the results from an >analysis. Can anybody tell me how SPM calculates the corrected p-value? Thanks, Hong