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Hi - if you are taking out the subject means (which makes sense) then your concern is probably correct, namely that you need to re-state the dummy variables such that each is zero-mean. Obviously this will change the meaning of the group-level EVs and you will need different contrasts as well, but I'm sure you'll be able to ask the questions you want. A minor query on your contrasts - I'm not sure why you included subject mean values in the "dual task" contrast. Cheers. On Tue, 27 Jan 2004, Paige Scalf wrote: > Hi FSL Guru's > > I've painted myself into a really complicated repeated measures, > nested design, and I'd appreciate some feedback about whether the > EV's and contrasts I've set up for my second level analysis are > correct. > > This is a dual-task task visual attention study, so we have each > of the dual task conditions in a single task form. Because we > have some repeated measures within the dual task condition, > however, we end up with an unbalanced (nested design). > The conditions are > > A (single task) > 1 center task > 2 peripheral vision task, proximal to fixation > 3 peripheral vision task, distal to fixation > > B (dual task) > 1 center task + proximal peripheral task > 2 center task + distal peripheral task > > > I've set up my matrix using dummy variables, such that > > X1 = 1 single, 0 else > X2 = 1 center single, 0 else > X3 = 1 proximal peripheral single, 0 else > X4 = 1 proximal dual single, 0 else > X5 = subject 1 > X6 = subject 2 > X7 = subject 3 > > > The EV Matrix ends up being > > A1 1 1 0 0 1 0 0 > A2 1 0 1 0 1 0 0 > A3 1 0 0 0 1 0 0 > A4 0 0 0 1 1 0 0 > A5 0 0 0 0 1 0 0 > > And my contrasts are structured > > single task 1 0 0 0 0 0 0 > dual task 0 0 0 .5 .165 .165 .165 > center single 0 1 0 0 0 0 0 > etc. > > > My biggest concern is that my EV's are set up using a very simple > "on/off" matrix, but if I set it up in the more traditional manner > (i.e. 1 for single task, -1 for dual task) I find I can't make the > maths work out for the contrasts I need. (I'll be happy to accept > feedback about any errors, however). > > Thanks in advance for any help you can give. > > Paige > 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