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Subject:

Re: Conjunctions and thresholds

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Date:

Wed, 07 Jul 1999 10:38:21 +0100

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 ```Cathy, Many thanks for your speedy reply to my query. However, your answer has raised further questions in my mind. When I run the conjunction analysis I obviously get the results table with areas of activation and associated p values at the voxel and cluster level.. My question is therefore at what level of p value do the results become "significant" in a conjunction. I.e. if the corrected p value for an activation is p=0.24 and the uncorrected p<0.001 is it "valid" to report this as a "significant" result because it has survived a conjunction of 12 subjects at Karl's suggested threshold (from your reply) even though I had no a priori hypothesis for this location? Many thanks again. Mark Cathy Price wrote: > 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 > >======================== -- ======================== Dr. Mark Daglish Clinical Research Fellow Psychopharmacology Unit ======================== %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% ```