You are using a different contrast. You are testing if group1 is different from group2, this is different than testing whether the groups are different.

If you use the F-contrast (3 rows - not three separate contrasts):
1 -1 0 0
0 1 -1 0
0 0 1 -1

You will get the correct degrees of freedom.

If you enter each of them separately, then your df will be 1,94 as the test is between 2 groups, not 4 groups.


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 Mon, Feb 27, 2012 at 11:37 AM, Maria Serra <[log in to unmask]> wrote:
Dear Donald,

thank you for your answer. However, although I use the contrasts which you suggest me, degree(s) of freedom continue being the same (namely 1,94). I attach an image in order to show it to you.

In the other hand, If you include a 0 in the contrast, I understand that the effect of the group in that position is annulled, isn't it?


thank you in advance,




On Mon, Feb 27, 2012 at 4:58 PM, MCLAREN, Donald <[log in to unmask]> wrote:
For 4 groups, the ANOVA test for the main effect of group is:
1 -1 0 0
0 1 -1 0
0 0 1 -1

Using this will give you the df: 3,94.

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
=====================
This e-mail contains CONFIDENTIAL INFORMATION which may contain PROTECTED
HEALTHCARE INFORMATION and may also be LEGALLY PRIVILEGED and which is
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 (773)
406-2464 or email.




On Mon, Feb 27, 2012 at 9:04 AM, Maria Serra <[log in to unmask]> wrote:
Dear SPMers,

I have a statistic doubt regarding degrees of freedom showed in an ANOVA analyses performed by SPM (VBM). Why the degree(s) of freedom are 1, 94 instead of 3, 94, If I have 4 groups to compare?

Thank you in advance,

--
Maria Serra

PIC (Port d'Informació Científica)
Campus UAB, Edifici D
E-08193 Bellaterra, Barcelona
Telf. +34 93 586 8232
www.pic.es

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--
Maria Serra

PIC (Port d'Informació Científica)
Campus UAB, Edifici D
E-08193 Bellaterra, Barcelona
Telf. +34 93 586 8232
www.pic.es

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Please consider the environment before printing this e-mail