Suppose that the response variable y is related to the explanatory variables x1 and x2 by the regression equation
y̅ = 6 - 0.3x1 + 1.7x2
(a) Construct a graph similar to Figure 21 showing the relationship between the expected value of y and x1 for x2 =10, 20, and 30.
(b) Construct a graph showing the relationship between the expected value of y and x2 for x1 = 40, 50, and 60.
(c) How can we tell from the graphs alone that there is no interaction between x1 and x2?
(d) Redo parts (a) and (b) with the interaction term 0.04x1x2 added to the regression equation. How do the graphs differ?