Hi - you'll need to do the 'anova maths' to check this I'm afraid:
sometimes apparently equivalent models/contrasts are what they appear;
other times, things can be counterintuitive.
Cheers
--------------------
Stephen M. Smith, Professor of Biomedical Engineering
Associate Director, Oxford University FMRIB Centre
FMRIB, JR Hospital, Headington,
Oxford. OX3 9 DU, UK
+44 (0) 1865 222726 (fax 222717)
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http://www.fmrib.ox.ac.uk/~steve
----------------------
On 13 Aug 2009, at 14:43, Colm Connolly <[log in to unmask]> wrote:
>
> On 4 Oct 2008, at 09:59, Thomas Nichols wrote:
>
>> Setting up a fourth variable would lead to a linearly-dependent
>> design matrix. I.e. the sum of EV2, EV3 and (your proposed) EV4
>> would equal EV1. If you dropped the grand mean covariate, you
>> could take the approach you suggest, and, in fact, that's the dummy
>> variable approach taken inhttp://www.fmrib.ox.ac.uk/fsl/feat5/
>> detail.html#FTests .
>
>
> Sorry to resurrect an old thread.
>
> I'm doing similar thing as Ben at the moment and am wondering why
> you would choose the grand-mean approach to specifying the EVs and
> contrasts versus the (simpler to my mind) approach of using the
> dummy variable?
>
> If I under stand this correctly (assuming n=3 and there are 3 groups
> and no repeated measures)
> long mid short age
> 1 0 0 12
> 1 0 0 11
> 1 0 0 13
> 0 1 0 11
> 0 1 0 17
> 0 1 0 14
> 0 0 1 10
> 0 0 1 12
> 0 0 1 16
>
> and contrasts (* indicates inclusion in an F-test)
> 1 0 0 0 *
> 0 1 0 0 *
> 0 0 1 0 *
> 1 -1 0 0 #long > mid
> 0 1 -1 0 #mid > short
>
> will allow me to ask the question "is there a group effect (the F-
> test)" and then to tweeze apart some of the possible pair-wise
> comparisons having accounted for variance attributable to age. Am I
> correct?
>
> Regards,
> --
> Dr Colm G. Connolly
> Institute of Neuroscience
> The Lloyd Building
> University of Dublin
> Trinity College, Dublin 2, Éire
> Fax: +353-1-671-3183
>
> Please note that electronic mail to, from or within the Trinity
> College Dublin, may be the
> subject of a request under the Freedom of Information Act.
>
>
>
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