Why does the interaction between a squared term and a main effect always correlate with the main effect?
I am setting up a custom experimental design with three factors, two continuous and one categorical. I want to look at main effects, one way interactions and a squared term for one of the continuous factors, this is all fairly straightforward. What I also want to look at is the interaction between the main terms and the squared term. To me this means how the level of a given factor affects the for...