In a semiconductor manufacturing process, CVD metal thickness was measured on 30 wafers obtained over approximately two weeks. Data are shown in the following table.

(a) Using all the data, compute trial control limits for individual observations and moving-range charts. Construct the chart and plot the data. Determine whether the process is in statistical control. If not, assume that assignable causes can be found to eliminate these samples and revise the control limits.

(b) Estimate the process mean and standard deviation for the in-control process.

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Wafer Wafer 16 1 16.8 15.4 2 14.9 17 14.3 3 18.3 18 16.1 4. 16.5 19 15.8 17.1 20 15.9 15.2 17.4 21 15.9 22 16.7 8. 14.4 23 15.2 9. 15.0 24 14.7 10 15.7 25 17.9 17.1 11 26 14.8 15.9 12 27 17.0 13 16.4 28 16.2 14 15.8 29 15.6 15 15.4 30 16.3