Suppose that the variables y, x1, and x2 are related by the regression model y 5 1.8 1 .1x1 1 .8x2 1 e
a. Construct a graph (similar to that of Figure
14.5) showing the relationship between mean y and x2 for fixed values 10, 20, and 30 of x1.
b. Construct a graph depicting the relationship between mean y and x1 for fixed values 50, 55, and 60 of x2.
c. What aspect of the graphs in Parts (a) and (b) can be attributed to the lack of an interaction between x1 and x2?
d. Suppose the interaction term .03x3 where x3 = x1x2 is added to the regression model equation. Using this new model, construct the graphs described in Parts (a) and (b). How do they differ from those obtained in Parts (a) and (b)?

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