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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). > > >