Robin,
> Perhaps a basic question about setting up contrasts, but I'm confused.
>
> I have, for example, 3 conditions A, B, and C, one ev for each. B is a repeat presentation of A, but I
> broke them appart to be able to probe differnces to repeats. I would like to perform the following
> contrasts --
>
> A + B
>
> (A + B) - C
>
> The first, I assume, is just 1 1 0. I also assume this would be the same as having had 2 evs, the
> first ev = A+B with the contrast 1 0. Is this true?
Approximately yes. The stats will be a bit different due to
the extra degree of freedom being lost when you have 3 evs instead of 2
> As for the second contrast, is it 1 1 -1 or 1 1 -2?
1 1 -1 would ask is the effect of A plus B bigger than C
1 1 -2 would ask is the average effect of A and B bigger than C
(equivalent to 1/2 1/2 -1)
I guessing you want 1 1 -2
--
Cheers, Mark.
Mark Woolrich.
Oxford University Centre for Functional MRI of the Brain (FMRIB),
John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK.
Tel: (+44)1865-222782 Homepage: http://www.fmrib.ox.ac.uk/~woolrich
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