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Hello Mark According to Karl, the following heuristic can be applied. The threshold you need to chose for each subject (Pc) should equal the threshold you want for the conjunction (Pw), when Pc is raised to the power of n. So if the uncorrected threshold you want for the conjunction is p<0.001 then the threshold for each subject is 0.4217 for 8 subjects (approximately what you used) and 0.1778 for 4 subjects (higher than you used). i.e. >> 0.001^(1/4) = 0.1778 Hopefully this stricter threshold with the lower P value will give you more sensible results for the n=4 group. Cathy >Dear SPMers, > >I have a data set of 12 subjects who had 12 run water PET scans. The >subjects split into 2 groups (n=8 & 4 for the groups). I would like to >see which areas of activation are common to the subjects in each group. >I therefore want to do conjunction analyses for each group of subjects. >My question concerns appropriate thresholding (p values) for the results >from these conjunctions. > >I have entered the data using the subject x condition interaction design >for PET studies with each subject having 6 scans under each condition. >I have then done a separate contrast for each subject (-1 1). I then >obviously do a conjunction of these individual contrasts including the 4 >or 8 subjects I am interested in. I have no a priori hypotheses of >particular areas that I am expecting differences so suspect that I >should be using corrected p values. > >I have specified low thresholds for the contrast conjunctions (eg. >p<0.5) which for 8 subjects give fairly discrete activations in sensible >places, but for 4 subjects most of the brain is pale gray. Intuitively >I can understand that the fewer subjects I include in a conjunction the >more likely activations will be common in the conjunction. What I do >not understand is some way to quantify this so that I do not make type I >or type II errors interpreting the conjunctions. > >Help on this would be appreciated. > >Mark > > >======================== >Dr. Mark Daglish >Clinical Research Fellow >Psychopharmacology Unit >University of Bristol >======================== %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%