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Bruno, Here are some thoughts: (1) You could use small-volume correction -- this will correct for regions that were significant. This method uses the pooled error term and spatial smoothness of the data to your advantage. (2) A statistician might say you should correct for the number of post-hoc tests, but people have argued that since you have many more brain voxels that comparisons, correcting for tests isn't necessary. However, I still think you need to correct for the number of tests. (3) If we combine these two thoughts. For regions that are significant, we want assess what are the pair-wise differences. Since we are only testing those regions, it makes sense to use small-volume corrections (+correcting for the number of tests). If you use a cluster extent and cluster p-value, you could use any number of mutliple-comparisons for correcting for the number of tests. I never got a good answer to whether the voxel p-values should be corrected for the number of tests or if the clusters p-values should be corrected for the number of tests. Doing voxel correction changes the results. Best Regards, Donald McLaren ================= D.G. McLaren, Ph.D. Postdoctoral Research Fellow, GRECC, Bedford VA Research Fellow, Department of Neurology, Massachusetts General Hospital and Harvard Medical School Office: (773) 406-2464 ===================== This e-mail contains CONFIDENTIAL INFORMATION which may contain PROTECTED HEALTHCARE INFORMATION and may also be LEGALLY PRIVILEGED and which is intended only for the use of the individual or entity named above. If the reader of the e-mail is not the intended recipient or the employee or agent responsible for delivering it to the intended recipient, you are hereby notified that you are in possession of confidential and privileged information. Any unauthorized use, disclosure, copying or the taking of any action in reliance on the contents of this information is strictly prohibited and may be unlawful. If you have received this e-mail unintentionally, please immediately notify the sender via telephone at (773) 406-2464 or email. On Fri, Feb 3, 2012 at 12:29 PM, Bruno L. Giordano <[log in to unmask]>wrote: > Hello, > > I am analyzing a factorial design. After FWE correction, I obtained a > significant main effect of a 3-levels factor (F test). I would like to run > some post-hoc contrasts within the significant-F region (a few clusters), > to test for pairwise differences between the various levels of the design > factor. > > My question: is it appropriate to carry out the post-hoc contrast by using > the same FWE correction (computed considering the entire brain) when my > interest is in a rather restricted area? Don't I run the risk of being too > conservative and/or imprecise? > > Can someone please suggest the standard approach in this case? Should one > re-estimate the proper FWE correction by re-running the factorial model > within the regions of interest? > > Cheers, > > Bruno >