Use the computer output from Problem 12 in Chapter 8 and the output given hereto answer the following questions.
a. State the model that relates change in refraction (Y) to baseline refraction (X1), baseline curvature (X2), and the interaction of X1 and X2. Is the partial F test for the interaction significant?
b. Is it appropriate to assess confounding given your answer to part (a)? Explain.
c. If your answer to part (b) is yes, does X2 confound the relationship between Y and X1?
d. If your answer to part (b) is yes, does X1 confound the relationship between Y and X2?
e. In light of your answers to parts (a) through (d), and on considerations of precision, which predictor(s) should be included in the model?

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Regression of Y on X1, X2, and X1X2 Parameter Intercept X1 X2 X1X2 lPortion of output omitted Estimate 25.85215663 2.69854313 Standard Error t ValuePr>It 16.33395716 3.41991591 0.37006537 1.58 0.1199 0.79 0.4339 -1.42 0.1613 0.52636869 0.08778420.07750834-087 Regression of Y on X1 Portion of output omitted Parameter Intercept X1 Estimate Standard Error Value Pr>It 2.619102900 041260537 6.35 0.0001 0 2987293 328 0.09463300 -3,16 0.0027 Regression of Y on X2 IPortion of output omitted Parameter Intercept X2 Estimate Standard Error tValue Pr> It 256 00135 187 00674 14.14611679 5.52569396 -0.23446227 0 0.12544713 Regression of Y on X1 and X2 Source Model Error Corrected Total DF Sum of Squares F Value Pr>F 7.02 00021 2 1753454762 955709 52 79.95410472 20 82.41 R-SquareCoeff ry Mean 0219308 29 22453 3.82320755 Parameter Intercept X1 X2 Portion of output omitted EstimateStandard Error t Value Pr It 12 29334748 -0.291353960.09241113 3.15 -0.21905406 011581741 9 0.0644 5.13074580 2.40 0.0204 3.15 0.0027