If a and b are factor variables are there any differences between ~ a:b ~ b:a ~ a %in% b ~ b %in% a playing with model.matrix doesn't show any differences in the resultant matrices, which is what I expect based on my understanding if what's being specified. I'm hoping that someone can clarify what (if any) the differences are. Thanks, Dan -- Daniel Lakeland dlakelan at street-artists.org http://www.street-artists.org/~dlakelan