3.27. Refer to the SENIC data set in Appendix C. I and Project 1.45. a. For each of the three fitted regression models, obtain the residuals and prepare a residual plot against X and a normal probability plot. Summarize your conclusions. Is linear regression model (2.1) more apt in one case than in the others? b. Obtain the fitted regression function for the relation between length of stay and infection risk after deleting cases 47 ( X47 = 6.5, Y47 = 19.56) and 112 (X112 = 5.9, Y112 = 17.94). From this fitted regression function obtain separate 95 percent prediction intervals for new Y observations at X = 6.5 and X = 5.9, respectively. Do observations Y47 and Y112 fall outside these prediction intervals? Discuss the significance of this. 3.28. Refer to the SENIC data set in Appendix C. 1 and Project 1.46. For each geographic region, obtain the residuals and prepare a residual plot against X and a normal probability plot. Do the four regions appear to have similar error variances? What other conclusions do you draw from your plots