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

Best Regards, Donald McLaren
D.G. McLaren
University of Wisconsin - Madison
Neuroscience Training Program
Office: (608) 265-9672
Lab: (608) 256-1901 ext 12914
This e-mail contains CONFIDENTIAL INFORMATION which may contain PROTECTED
intended only for the use of the individual or entity named above. If the
reader of the e-mail is not the intended recipient or the employee or agent
responsible for delivering it to the intended recipient, you are hereby
notified that you are in possession of confidential and privileged
information. Any unauthorized use, disclosure, copying or the taking of any
action in reliance on the contents of this information is strictly
prohibited and may be unlawful. If you have received this e-mail
unintentionally, please immediately notify the sender via telephone at (608)
265-9672 or email.