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Steve, I just now realized that when saying PE vs COPE I in fact meant (CO)PE vs ZSTAT. Just to make sure I understand, zstat is COPE-mean(COPE)/sqrt(VARCOPE)? my point is that I compare patterns of activations across different EVs, and some are collected in separate runs (with a run being machine ON to machine OFF), hence comparing measures that do not account for differences in signal mean and variability across runs I may be getting non-interpretable results. cheers martin Steve Smith wrote: > Hi Martin, > > On 31 Oct 2006, at 20:35, Martin M. Monti wrote: > >> Dear Lords of FSL, >> >> I have a simple question re the contents of the PE and COPE images. >> >> At a first level (single sub - single run) PE1 is a matrix containing >> the GLM parameter estimate (b) for each voxel for EV1. If I created a >> contrast with (say) a 1 in EV1 and zeros in all other EVs I would >> then get COPE1 to be in fact the same thing as PE1 except it's a beta >> as opposed to a non standardized estimate (i.e. b). [the betas are >> what I need..] >> >> So far so good (right? ..I think we previously discussed this already..) > > Not quite - in this case (contrast [1 0 0...]) then COPE1 is _exactly_ > PE1 - neither is standardised in any way at this point. PE1 is the > beta, and for this contrast COPE1=PE1. > > I'm not sure what distinction you're making between b and beta. In the > GLM: > Y=Xb+e > Then b (ie beta) is the parameter estimates. > >> Now, say that I have multiple runs and multiple subjects, and all I >> do is average (i.e. no subtractions) EV1 across multiple sessions >> (for each given subject) and across multiple subjects. >> This group analysis will yield again a PE1 and a COPE1 (obtained >> again just by creating a contrast with 1 in all EV1s and 0 for all >> other EVs). >> >> Are the EP1 and COPE1 matrices just (group) average bs and (group) >> average betas, respectively? > > Apart from the fact that the "bs" _are_ the betas, then yes, the rest > is correct. > > Cheers, Steve. > > >> >> cheers >> Martin >> >> >> -- >> >> >> >> -------------------------------------------- >> "Upward, not Northward!" [Edwin Abbott - FlatLand] >> I became insane, with long intervals of horrible sanity. [E.A. Poe] >> Would you tell me, please, which way I ought to go from here? ''That >> depends a good deal on where you want to get to.'' Said the Cat. I >> don't much care where -- Said Alice. Then it doesn't matter which way >> you go, said the Cat. [Lewis Carroll] >> Che si trattasse di una gara? ..Un colpo il mondo e un colpo tu, a >> chi stupiva un po' di piu'.. [Per Un Vecchio Bambino - Roberto >> Vecchioni] >> -------------------------------------------- >> Martin M. Monti >> Princeton University >> Department of Psychology >> Green Hall 3-S-8 >> (609) 258-5679 >> -------------------------------------------- > > > --------------------------------------------------------------------------- > > Stephen M. Smith, Professor of Biomedical Engineering > Associate Director, Oxford University FMRIB Centre > > FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK > +44 (0) 1865 222726 (fax 222717) > [log in to unmask] http://www.fmrib.ox.ac.uk/~steve > --------------------------------------------------------------------------- > > -- -------------------------------------------- "Upward, not Northward!" [Edwin Abbott - FlatLand] I became insane, with long intervals of horrible sanity. [E.A. Poe] Would you tell me, please, which way I ought to go from here? ''That depends a good deal on where you want to get to.'' Said the Cat. I don't much care where -- Said Alice. Then it doesn't matter which way you go, said the Cat. [Lewis Carroll] Che si trattasse di una gara? ..Un colpo il mondo e un colpo tu, a chi stupiva un po' di piu'.. [Per Un Vecchio Bambino - Roberto Vecchioni] -------------------------------------------- Martin M. Monti Princeton University Department of Psychology Green Hall 3-S-8 (609) 258-5679 www.webmartin.net --------------------------------------------