Hi,

I have two within subjects conditions, each one of them with 3 levels: cond1 (light,medium, dark), cond2 (F1,F2,F3). I want to test the effect of each individually and their interaction. I think I am ok with the subjects, condition1 and 2 but I am struggling to add into the design matrix the interaction effect (condition 1 * condition 2). Any help will be much appreaciated. 

For simplicity, we will assume 3 subjects:

                                            cond1  cond2        interaction
S1 Light F1         1    0    0    0    0    0    0             ??
S2 Light F2         0    1    0    0    0    0    1            
S3 Light F3         0    0    1    0    0    1    0            
S1 Medium F1    1    0    0    0    1    0    0        
S2 Medium F2    0    1    0    0    1    0    1    
S3 Medium F3    0    0    1    0    1    1    0    
S1 Dark F1          1    0    0    1    0    0    0    
S2 Dark F2          0    1    0    1    0    0    1    
S3 Dark F3          0    0    1    1    0    1    0    

Contrast for condition1: [0,0,0,1,1,0,0,0,0]

Contrast for condition2:[0,0,0,0,0,1,1,0,0]

Contrast for interaction [0,0,0,0,0,0,0,1,1]

And exchange blocks: [1,2,3,1,2,3,1,2,3,1,2,3]

Thank you for your help!