dear FSL people,
I have what I think is a basic question (*very* basic, sorry about that...) which came to my mind while digging in the posts about interaction effects in multiple regression, and I would appreciate a lot to have your advice on it.
Suppose I have the following model:
Yhat = b0 + b1*X + b2*Z + error
and I want to test for an interaction between X and Z. Now, what is the difference between:
1. using an interaction EV
Yhat = b0 + b1*X + b2*Z + b3*(X.*Z)+ error
and contrast matrix [0 0 0 1]
2. evaluating the difference in the regression slopes with the contrast matrix [0 1 -1] and [0 -1 1] in the original model
thanks a lot for your reply,
all the best!
leonardo
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