What does the term X1*X1 mean in an RSM design?
Hello, When I'm using DOE and I click 2nd order interactions, the model will add combinations of every interaction of my factors, e.g. X1*X2, X2*X3 and X1*X3. This makes sense to me because these terms tell me about the combined effect of two factors on my response. However, if I click the RSM button it will add quadratic terms such as X1*X1, X2*X2 and X3*X3. My question is, what physical meaning ...