Dear SPM experts,
I'd like to do a 2nd level one-way ANOVA with a within subject factor, as introduced with the SPM8 updates 4010. I have 11 subjects and a 4 experimental conditions. I can set up the model just fine but I'm not sure if I'm doing the contrasts right:
I noted that for this type of model, SPM accepts only contrasts which sum up to zero. To compare each of the conditions against the other ones I use 4 separate t-contrasts:
1: 0 0 0 0 0 0 0 0 0 0 0 3 -1 -1 -1
2: 0 0 0 0 0 0 0 0 0 0 0 -1 3 -1 -1
3: 0 0 0 0 0 0 0 0 0 0 0 -1 -1 3 -1
4: 0 0 0 0 0 0 0 0 0 0 0 -1 -1 -1 3
For the main effect over all 4 conditions, I specify an F-contrast containing the above 4 contrast rows.
So here are my questions:
1. Are the above contrasts correct.
2. Why do the contrasts have to sum up to 0? I thought the F-contrast should be [zeros(4, 11) eye(4)], but SPM says "invalid contrast".
3. Can I simply multiply my contrasts by -1 to test for "negative" effects?
Thanks for your help, Adrian
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