15-8. The copper content of a plating bath is measured three times per day, and the results are reported in ppm. The and r values for 25 days are shown in the following table: Day I Day 1 5.45 1.21 14 7.01 1.45 2 5.39 0.95 15 5.83 1.37 3 6.85 1.43 16 6.35 1.04 4 6.74 1.29 17 6.05 0.83 5 5.83 1.35 18 7.11 1.35 6 67 7.22 0.88 19 7.32 1.09 7 6.39 0.92 20 5.90 1.22 8 6.50 1.13 21 5.50 0.98 9 7.15 1.25 22 6.32 1.21 10 5.92 1.05 23 6.55 0.76 11 6.45 0.98 24 5.90 1.20 12 5.38 1.36 25 5.95 1.19 13 6.03 0.83 (a) Using all the data, find trial control limits for X and R charts, construct the chart, and plot the data. Is the process in statistical control? (b) If necessary, revise the control limits computed in part (a). assuming that any samples that plot outside the control lim- its can be eliminated.