Donald -
When you use those contrast matrices for F-tests within an analysis, you
should indeed get the same results. This is because they are
mathematically equivalent, in that they both span the 2-df space defined
by all possible differences between 3 datapoints (ie both have rank 2).
My emails below concerned contrast vectors that one might evaluate at
the 1st-level (creating one contrast image per row of your matrices),
which you then use as the data for a 2nd-level analysis (in which you
use the F-contrast [1 0; 0 1] on the 2dfs of the differences). In this
situation, you are likely to get different answers with your two
matrices, because the rows have different scalings and correlations (in
fact, neither is correct - you need to create orthonormal contrast
weights using "orth(diff(eye(3)))").
Sorry if it is confusing
Rik
MCLAREN, Donald wrote:
> Rik,
>
> You've had several previous posts on the issue of contrast for factors
> with more than 2 levels.
>
> http://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=ind05&L=SPM&D=0&I=-3&P=259440
> <http://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=ind05&L=SPM&D=0&I=-3&P=259440>
>
> and
>
> http://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=ind05&L=SPM&D=0&I=-3&P=259731
> <http://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=ind05&L=SPM&D=0&I=-3&P=259731>
>
> Both of them make sense; however, when I use either of the following
> contrasts:
> [1 -1 0; 0 1 -1] or [-2 1 1; 0 1 -1]
>
> I get the same results.
>
> Has the error mentioned in your previous posts been eliminated in SPM5
> (e.g. does SPM5 adjust the [1 -1 0; 0 1 -1] to make it produce the
> same results as the latter contrast)?
>
> Thanks.
>
> --
> Best Regards, Donald McLaren
> =====================
> D.G. McLaren
> University of Wisconsin - Madison
> Neuroscience Training Program
> Tel: (773) 406 2464
> =====================
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--
----------------------------------------
Dr Richard Henson
MRC Cognition & Brain Sciences Unit
15 Chaucer Road
Cambridge
CB2 7EF, UK
Tel: +44 (0)1223 355 294 x522
Fax: +44 (0)1223 359 062
http://www.mrc-cbu.cam.ac.uk/~rik.henson
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