It would be the same as your 2nd matrix with the right four columns negated.
doug
Stefan Brauns wrote:
Hi all,
I am trying to examine the interaction of 2 factors (2 x 4 groups) and have difficulties to find the right contrast matrix for mri_glmfit.
We have patients and controls at four different aquisition sites and want to know if the changes (i.e. thickness) between patients and controls are equal in every site.
so we have 8 groups:
patientssite1 patientssite2 patientssite3 patientssite4 controlssite1 controlssite2 controlssite3 controlssite4
the contrast for differences between patients and controls (T-Test) would be:
1/4 1/4 1/4 1/4 -1/4 -1/4 -1/4 -1/4
the contrast to investigate if the observed measured value is equal in all sites (F-Test) would be:
1/2 -1/2 0 0 1/2 -1/2 0 0 0 1/2 -1/2 0 0 1/2 -1/2 0 0 0 1/2 -1/2 0 0 1/2 -1/2
How would the contrast matrix look like if I want to test if there is an overall interaction between site and diagnosis, i.e. does the effect of diagnosis vary by site (F-Test)?
Many thanks, Stefan
-- Stefan Brauns
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