Thirty observations on concentration (in g/l) of the active ingredient in a liquid cleaner produced in a continuous chemical process are shown in Table 6E.26.

(a) A normal probability plot of the concentration data is shown in Figure 6.29. The straight line was fit by eye to pass approximately through the 20th and 80th percentiles. Does the normality assumption seem reasonable here?
b) Set up individuals and moving range control charts for the concentration data. Interpret the charts.
(c) Construct a normal probability plot for the natural log of concentration. Is the transformed variable normally distributed?

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TABLE 6 E.26 Data for Exercise 6.68 Obseryation Concentration Observation Concentration 604 69.5 784 72.8 78.2 78.7 56.9 78.4 796 100.8 99.6 649 755 704 68.1 16 17 18 19 20 21 23 24 25 26 27 28 29 30 99.9 59.3 60.0 74.7 75.8 76.6 68.4 83.1 61.I 54.9 69.1 67.5 69.2 87.2 73.0 L1 13 14 15