An article in Solid State Technology (May 1987) describes an experiment to determine the effect of C2F6 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.4.

(a) Does C2F6 flow rate affect etch uniformity? Answer this question by using an analysis of variance with = 0.05.
(b) Construct a box plot 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 of etch uniformity (a small percentage is best).
(c) Plot the residuals versus predicted C2F6 flow. Interpret this plot.
(d) Does the normality assumption seem reasonable in this problem?

TABLE 4E.4 Uniformity Data for Exercise 4.35 Observations C2 Fo Flow SCCM 235 6 125 2.1 2.6 4.6 3.2 3.0 3.8 160 4.6 49 50 42 36 4.2 4.6 2.9 34 3.5 4.1 5.1 200