Consider an example in which there are two control variables x and x and a single noise variable z. The goal is to determine the levels of x and x that are robust to changes in z, i.e., levels of x and x that minimize the variance produced in the response y as z moves between –1 and +1. The variables x and x are at two levels, –1 and +1, in the experiment. The data produce the plots in Figure 15.22 above. Note that x and x interact with the noise variable z. What settings on x and x (–1 or +1 for each) result in minimum variance in y? Explain.