Interaction terms are easy to add using our software. Assuming you are working with CBC within our SSI Web system, you click Analysis + "HB"...then you click the Settings button...then you click Attribute Coding. At the bottom of the dialog, click the green "plus" icon to add interaction terms to the model. Pick two attributes from that dialog to interact.
Collapsing two binary attributes into a single attribute is done (with some data processing work on the .CSV file that you can export from SSI Web of the CBC data...it's the format for raw CBC data to take to our standalone CBC/HB system for HB estimation)...as follows:
Assume you had two 2-level attributes. The number of attribute level combinations between those two is four: 1,1...1,2...2,1...2,2. So, think about a single 4-level attribute that encodes the information you previously had specified as two 2-level attributes. The mapping is as following:
Level 1 of the collapsed attribute is coded when the 1,1 combination occurs
Level 2 of the collapsed attribute is coded when the 1,2 combination occurs, etc.
Please note that including the two separate binary attributes plus the interaction between them in the model is mathematically identical as recoding into a single attribute with 4 levels.