Linear regression alayses with interaction effect of two categorical variables

#1
Dear all,

I have to run a linear regression analysis in SPSS with an interaction effect of two categorical variables:

- Modality (audio, visual and audio-visual)
- Repetition (1x, 2x and 4x)

I have already dummify the variables. Before I can run the analysis 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). Does someone know the formula that I have to enter?

I hope that someone can help me, thank you in advance :D!
 

hlsmith

Omega Contributor
#2
This seems like an SPSS issue. In most programs you can just enter a multiplicative term into the model (modality*repetition).
 
#3
Thanks a lot :)! I have two independent variables that have three categories (so two dummy variables for modality and also two for repetition):

Modality_Dummy1
Modality_Dummy 2
Repetition_Dummy1
Repetition_Dummy2

I don't know how I have to enter the multiplicative term. Do you think that I have to include more interaction terms?
 
#4
Thanks a lot ! I have two independent variables that have three categories (so two dummy variables for modality and also two for repetition):

Modality_Dummy1
Modality_Dummy 2
Repetition_Dummy1
Repetition_Dummy2

I don't know how I have to enter the multiplicative term. Do you think that I have to include more interaction terms?