 |
 |
Hi, yes that all makes sense. Approach 2 is what you want and you're right to take out the constant-EV ev1 from your original email. Cheers. On Tue, 31 Aug 2004, Xun Liu wrote: > On Tue, 31 Aug 2004 21:17:33 +0100, X Liu <[log in to unmask]> wrote: > > >Concatenate run/session or not? > > > >We have a design with 2 independent variables (IV1 and IV2), each with 3 > >levels. IV1 is manipulated across blocks within a run and IV2 is > >manipulated across runs. There are two approaches I can think of to anaylze > >the data for the main effects and interaction for this design. What is the > >advantage and disadvantage of each approach, from the conceptual and > >practical perspectives? Is one more valid than the other from the > >statistics point of view? Thanks very much. > > > > > >Approach 2: > >Analyze each run separately and model just the 3 conditions/levels of IV1 > >(ev1, ev2, ev3). Then the main effects of IV1 can be set up as below. > > > >Contrast ev1 ev2 ev3 > >mean (1) 1 1 1 > >IV1 (2) 1 -1 0 > > (3) 1 0 -1 > > (4) 0 1 -1 > > > >And then proceed to the group analysis with the EVs/contrasts setup as > >below for the three levels of IV2 (say for 5 subjects) > > > >Group ev1 ev2 ev3 ev4 ev5 ev6 ev7 ev8 > >1 1 1 1 1 0 0 0 0 > >1 1 1 1 0 1 0 0 0 > >1 1 1 1 0 0 1 0 0 > >1 1 1 1 0 0 0 1 0 > >1 1 1 1 0 0 0 0 1 > >1 1 -1 0 1 0 0 0 0 > >1 1 -1 0 0 1 0 0 0 > >1 1 -1 0 0 0 1 0 0 > >1 1 -1 0 0 0 0 1 0 > >1 1 -1 0 0 0 0 0 1 > >1 1 0 -1 1 0 0 0 0 > >1 1 0 -1 0 1 0 0 0 > >1 1 0 -1 0 0 1 0 0 > >1 1 0 -1 0 0 0 1 0 > >1 1 0 -1 0 0 0 0 1 > > > >Contrast ev1 ev2 ev3 ev4 ev5 ev6 ev7 ev8 > >c1 1 0 0 0 0 0 0 0 > >c2 0 1 0 0 0 0 0 0 > >c3 0 0 1 0 0 0 0 0 > >c4 0 1 -1 0 0 0 0 0 > > > >Then the .gfeat folder should include 4 cope#.feat subfolders, one for each > >of the contrasts from the first level. zstat2 to zstat4 of cope1.feat will > >assess the main effect of IV2 (zstat1 is the overall grand mean of both IVs > >again baseline). zstat1 of cope2.feat to cope4.feat will assess the main > >effect of IV1. zstat2 to zstat4 of cope2.feat to cope4.feat will assess the > >interactions. > > Just found out an error. To correct myself, ev1 is a linear combination of > ev4 to ev8. So take out ev1 and contrast 1 is just the combination of the > new ev3 to ev7 (weighted or not WRT to the other contrasts?). The > interpretation of the main effects and interaction above stands. > > Group ev1 ev2 ev3 ev4 ev5 ev6 ev7 > 1 1 1 1 0 0 0 0 > 1 1 1 0 1 0 0 0 > 1 1 1 0 0 1 0 0 > 1 1 1 0 0 0 1 0 > 1 1 1 0 0 0 0 1 > 1 -1 0 1 0 0 0 0 > 1 -1 0 0 1 0 0 0 > 1 -1 0 0 0 1 0 0 > 1 -1 0 0 0 0 1 0 > 1 -1 0 0 0 0 0 1 > 1 0 -1 1 0 0 0 0 > 1 0 -1 0 1 0 0 0 > 1 0 -1 0 0 1 0 0 > 1 0 -1 0 0 0 1 0 > 1 0 -1 0 0 0 0 1 > > Contrast ev1 ev2 ev3 ev4 ev5 ev6 ev7 > c1 0 0 0.2 0.2 0.2 0.2 0.2 > c2 1 0 0 0 0 0 0 > c3 0 1 0 0 0 0 0 > c4 1 -1 0 0 0 0 0 > Stephen M. Smith DPhil Associate Director, FMRIB and Analysis Research Coordinator Oxford University Centre for Functional MRI of the Brain John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK +44 (0) 1865 222726 (fax 222717) [log in to unmask] http://www.fmrib.ox.ac.uk/~steve