Thank you really much for your answer! I had suspicions with this
matrix, someone said that it would do the trick... I've now made new
ones. I didn't find any model in my version of spm where I could model
all interactions at the same time, do you know what this model would
look like? My categorical variables are both dichotomial, as you
thought. I did a matrix using one variable as condition and the other
two as covariates, and then I could model the interactions between the
condition and a covariate. This gives a matrix with seven columns (two
for the condition, then both covariates and both interactions, and
then a constant). So with two different matrices I should get all
interactions except for the one with all three variables... Does this
sound reasonable? How do I define the contrasts for this matrix?
Thank you again, and hopefully you can help me one more time!
Lainaus "Stephen J. Fromm" <[log in to unmask]>:
> Based on your description, I don't think you can model the
> interactions with that design matrix.
>
> The number of columns you need for the two categorical variables
> alone is the product of the number of levels in each (minus one if
> SPM automatically includes the constant regressor). If you also
> include a covariate like age, you need to multiply by an additional
> factor of two, if I recall correctly.
>
> The fact that your current design matrix (probably) models the main
> effects would indicate that your categorical variables each have two
> levels. If my claims above are correct, you'd need 2*2*2 = 8
> columns in the design matrix (if the constant isn't modeled).
>
>
>
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