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Depends on your question, the contrast below is comparing group1 to the average of groups 2-4.

An alternative is to ask the question: 
Does group1 differ from group2 OR group1 differ from group3 OR group1 differ from group4?

F= [1 -1 0 0; 1 0 -1 0; 1 0 0 -1]

Another alternative:
Do the groups differ from each other?
F=[1 -1 0 0; 0 1 -1 0; 0 0 1 -1]

On Thu, Mar 26, 2009 at 11:55 AM, Muhammad A Parvaz <[log in to unmask]> wrote:
hi all,

we are doing VBM using the VBM5 toolbox and we are doing an ANOVA with 4
groups. i was wondering what is the correct way to specify the contrast when
we want to compare one group against the other 3 groups. We are inclined to
use the following weights for our contrast

[-1 0.33 0.33 0.33]

is this right? will decreasing the weight of each group to 0.33 change the
signal and variance of that group in anyway?

Is there a better way to specify this contrast?

Thanks
-Muhammad



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Best Regards, Donald McLaren
=====================
D.G. McLaren
University of Wisconsin - Madison
Neuroscience Training Program
Office: (608) 265-9672
Lab: (608) 256-1901 ext 12914
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