Hello Donald and all,
It always struck me as odd that F-contrasts in SPM for a (one-way) ANOVA are specified this way (e.g. [1 -1 0; 0 1 -1]), as these two contrast vectors are non-orthogonal, wheras typical ANOVA's would use a full set of orthogonal contrasts to divide the "treatment" SS, e.g. [1 -1 0; 1 1 -2]. What am I missing here?
Thanks,
Andreas
>-----Original Message-----
>From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]] On
>Behalf Of MCLAREN, Donald
>Sent: 16 September 2011 06:18
>To: [log in to unmask]
>Subject: Re: [SPM] 1-way between-subs ANOVA contrast
>
>Use an F-Contrast:
>[1 -1 0 ; 0 1 -1]
>
>If this is significant, you can then evaluate the following t-contrasts:
>[1 -1 0]
>[1 0 -1]
>[0 1 -1]
>
>and the reverse contrasts:
>[-1 1 0]
>[-1 0 -1]
>[0 -1 1]
>
>Keep you eyes on the list to see if anyone responds about how to
>correct for the fact that you are doing 3 post-hoc tests, which
>inflates the type 1 error.
>
>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
>Office: (773) 406-2464
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>On Fri, Sep 16, 2011 at 12:58 AM, Adam Felton <[log in to unmask]> wrote:
>> I apologize in advance for asking such a simple question, but I couldn't find the
>direct answer in the archives and I couldn't figure it out from the ANOVA pdf's
>that have been previously recommended.
>>
>> How do I set the contrast weights to compare three groups with a one-way
>between-subjects ANOVA?
>>
>> I am comparing three groups (people who score better at task 1; people who
>score better at task 2; people who are equal at both tasks). I've seen it
>previously recommended that [1 -1 0] be used; is that equivalent to [-1 1 0] or [0
>-1 1], etc?
>>
>>
>> Thank you for your help,
>>
>>
>> Adam
>>
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