The copper content of a plating bath is measured three times per day, and the results are reported in ppm. The xÌ… and r values for 25 days are shown in the following table:

(a) Using all the data, find trial control limits for XÌ… 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 limits can be eliminated.

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Day Day 1.21 1 5.45 14 7.01 1.45 0.95 1.37 5.39 15 5.83 3 6.85 1.43 16 6.35 1.04 6.74 1.29 17 6.05 0.83 5 5.83 1.35 18 7.11 1.35 7.22 1.09 0.88 19 7.32 6.39 0.92 20 5.90 1.22 8. 6.50 1.13 21 5.50 0.98 1.25 22 9. 7.15 6.32 1.21 6.55 0.76 10 5.92 1.05 23 11 6.45 0.98 24 5.90 1.20 5.95 1.19 12 5.38 1.36 25 13 6.03 0.83