I have to run a linear regression analysis with an interaction effect of two categorical variables:
- Modality (audio, visual and audio-visual)
- Repetition (1x, 2x and 4x)
I have already dummified the variables. Before I can run the analyses I have to compute the interaction term. But I don't know how I have to do this in the case of a variables with 3 categories (Because I have two dummy variables for each independent variable).