An article in Solid State Technology (May 1987) describes an experiment to determine the effect of flow rate on etch uniformity on a silicon wafer used in integrated-circuit manufacturing. Three flow rates are tested, and the resulting uniformity (in percent) is observed for six test units at each flow rate. The data are shown in Table 4E.7.

a. Does flow rate affect etch uniformity? Answer this question by using an analysis of variance.
b. Construct a boxplot of the etch uniformity data. Use this plot, together with the analysis of variance results, to determine which gas flow rate would be best in terms or etch uniformity (a small percentage is best).
c. Plot the residuals versus predicted flow. Interpret this plot.
d. Does the normality assumption seem reasonable in this problem?

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CE Flow | Observations (SCCM 1 2 345 6 125 160 200 2.7 2.6 4.6 3.2 3.0 3.8 4.6 4.9 5.0 4.2 3.6 4.2 4.6 2.9 3.4 3.5 4.1 5.1