Continue with the data set in Table 18.5.3, representing the thicknesses of a protective coating.
a.* Find the center line for the R chart.
b.* Find the control limits for the R chart.
c. Draw the R chart.
d. Comment on what you see in the R chart. In particular, is the variability of this process in control? How do you know?

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TABLE 18.5.3 Thicknesses of a Protective Coating: 25 Samples of Three Items Each Individual Measurements within Each Sample Sample Identification Number 1251 12.60 12.40 12.44 12.78 12.75 12.53 12.64 12.57 12.70 12.74 12.90 13.05 12.88 13.03 12.96 12.91 12.95 13.17 13.17 12.95 1304 13.12 12.83 13.24 12.70 12.53 12.81 12.57 12.61 12.43 12.51 12.49 12.74 12.87 12.80 12.83 13.00 12.88 12.85 12.88 12.75 13.03 12.81 13.05 13.04 13.25 13.07 13.13 13.18 1257 12.56 12.60 12.61 12.68 12.51 1281 12.95 12.86 12.91 13.132 13.11 13.05 6 10 12 13 T4 15 16 17 18 13.01 12.89 13.17 12.97 12.80 12.95 13.11 1331 13.13 20 25