The tensile strength of Portland cement is being studied. Four different mixing techniques can be used economically; the results of an experiment to compare the mean strengths among these techniques is contained in the file "cement." (a) (5 points) Use ANOVA to test the hypothesis that mixing techniques have no effect on the strength of the cement. Use = 0.05. (b) (5 points) Plot the residuals versus predicted strength. What does the plot suggest regarding assumptions for our underlying ANOVA model. (c) (5 points) Construct a normal probability plot of the residuals. Is the normal error assumption for ANOVA met? (d) (5 points) Use Tukey’s HSD to determine which techniques have different mean strengths, if any. Use an overall = 0.05.