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Hi, A = a + b + c B = a -b C = a-c A+B+C = 3a So yes a [1 0 0 ] contrast gives you the mean of A,B,C. However if you just want to ignore cross-session variability you could just use an all 1s single EV and use the fixed-effects option? Cheers. On 24 Nov 2008, at 22:04, Brad Goodyear wrote: > Hi. > I want to compute the average across three conditions for each > subject by removing any > differences between the conditions. > For two conditions, I understand the EVs would be > > EV1 EV2 > 1 1 > 1 -1 > > would it not, and I compute the contrast (1,0) for the average > across the two conditions > with any differences removed? > > For three conditions, is it > > EV1 EV2 (a) EV3 (b) > 1 1 1 > 1 -1 0 > 1 0 -1 > > and then compute (1,0,0) since conditions A+B+C = a+b-a-b = 0, as > per the tripled t-test > example? > I then plan to average this (1,0,0) contrast across subjects to get > the mean across > condition with any differences between the conditions removed. > > -Brad > --------------------------------------------------------------------------- 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 ---------------------------------------------------------------------------