The uniformity of a silicon wafer following an etching process is determined by measuring the layer thickness at several locations and expressing uniformity as the range of the thicknesses. Table 6E.29 presents uniformity determinations for 30 consecutive wafers processed through the etching tool

(a) Is there evidence that uniformity is normally distributed? If not, find a suitable transformation for the data.
(b) Construct a control chart for individuals and a moving range control chart for uniformity for the etching process. Is the process in statistical control?

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. TABLE 6E.29 Uniformity Data for Exercise 6.71 Wafer Uniformity Wr Uniformity 18 19 12 14 18 21 13 12 10 26 12 13 14 23 14 12 21 21 30