Parts manufactured by an injection molding process are subjected to a compressive strength test. Twenty samples of five parts each are collected, and the compressive strengths (in psi) are shown in Table 6E.12.

(a) Establish x and R control charts for compressive strength using these data. Is the process in statistical control?
(b) After establishing the control charts in part (a), 15 new subgroups were collected and the compressive strengths are shown in Table 6E.13. Plot the x and R values against the control units from part (a) and draw conclusions.

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TABLE GE.12 Streneth Data for Exercise 6.21 Sample Number 1 83.0 呂1.2 78.7 75.7 77.0 791 7.3 2 88.6 78.3 78.8 710 84.2 80.2 17.6 85.7 758 4.3 75.2 810 80.4 10.4 4808 44 825 74.1 75.7 754 83.4 78.4 82.6 78.2 78.9 80.3 5.2 6 15.3 79.9 873 89.7 818 828 14.5 7 74.5 78.0 80.8 734 79. 77.3 7.4 8 79.2 84.4 81.5 86.0 43 81.1 14 9 80.5 86.2 16.2 64. 80.2 81.4 9.9 10 75.7 75.2 1.1 82.1 74.3 75.7 10.9 80.0 81.5 78.4 73.8 78. 784 7.7 12 80.6 81.8 79.3 73.8 81.7 79.4 8.0 13 82.7 8.3 9. 82.0 79.5 80.9 3.6 14 79.2 74.9 78.6 77.7 75.3 77.1 4.3 15 85.5 82.1 82.8 73.4 71.7 79.1 13.8 16 78.8 79.6 80.2 79.1 808 79.7 2.0 17 82.1 78.2 75.5 78.2 82. 79.2 6.6 18 845 16.9 83.5 812 79.2 811 76 1979.0 178 81.2 84.4 81.6 80.8 6.6 20 84.5 73.1 78.678.7 80.6 79.1 11.4 TABLE 6E.13 New Data for Exercise 6.21, part (b) Sample Number x 168.9 81.5 78.2 80.8 8.5 78.2 12.6 69.8 68.6 804 843 83.9 4 15.7 3 78.5 85.2 784 80.3 81.7 808 6.8 4 69 861 869 944 83.9 85.6 175 5 93.6 8L6 878 9.6 71.0 82.7 225 6 65.5 868 724 82.6 71.4 759 21.3 78.1 65.7 83.7 93.7 93.4 82.9 27.9 8 74.9 726 816 87.2 72.7 778 14.6 9 78.1 77.1 670 75.7 16.8 749 110 18.7 854 777 90.7 76.7 819 14.0 11 85.0 60.2 685 7.1 82.4 73.4 24.9 12 86.4 79.2 798 86.0 75.4 813 109 13 78.5 99.0 783 714 81.8 81.7 27.6 14 68.8 620 82.0 75 16.1 73.3 199 83.0 83.7 73.1 82.2 95.3 83.5 22.2 10 15