Dear Donald McLaren,
Because you have a between-subject design, the estimate (magnitude) of group 3 does not effect the magnitude of group 1 or 2 or their difference.
However, having group 3 in the model will effect the variance of the contrast and the t-statistic. Selecting assume equal variance or unequal variance will also effect the statistics.
Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Postdoctoral Research Fellow, GRECC, Bedford VA
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Website: http://www.martinos.org/~mclaren
Office: (773) 406-2464
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On Fri, Jun 15, 2012 at 4:35 AM, Martin Federbusch <[log in to unmask]> wrote:
---------- Forwarded message ----------
From: Martin Federbusch <[log in to unmask]>
Date: Fri, Jun 15, 2012 at 10:18 AM
Subject: T-contrast between 2 groups in a 3-group-design
To: [log in to unmask]
Dear all,
I am comparing 3 groups in a VBM-analysis (/2nd-level-analysis). When I'm using a T-contrast only between 2 of these groups, does SPM correct for example the degrees of freedom in this case?
e.g.: Group 1, 2 and 3 have 20 subjects each. All of these groups are in one design, so I have a design with 60 subjects.
Is it possible to use a T-contrast 1 -1 0, which would compare group 1 and 2, but leave out group 3 or do I have to do a new design with only 40 subjects?
Thank You in advance,
best,
Martin