A high-level voltage power supply should have a nominal output voltage of 350 V. A sample of four units is selected each day and tested for process-control purposes. The data shown in Table 6E.3 give the difference between the observed reading on each unit and the nominal voltage times ten; that is,
xi =(observed voltage on unit i€“350)10

(a) Set up x and R charts on this process. Is the process in statistical control?
(b) If specifications are at 350 V  5 V, what can you say about process capability?
(c) Is here evidence to support the claim that voltage is normally distributed?

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TABLE 6E.3 Voltage Data for Exercise 6.8 Sample 9 1015 2 0 46 11 8 10 5 6 13 7 13 6 12 10 10 5 910 7 6 0 8 7 5 10 4 8 12 1015 16 10 13 12 14 16 1311 9 13 15 14 7 110 12 10 16 16 15 1014 8 12 10 12 13 6 15 17 9 18 5 7101 19 20 13 14 9 12 15