The average time (minutes) that a customer has to wait for the arrival of a cab after calling the company has been observed for random samples of size 4. The data for 20 such samples are shown in Table 7-23. Previous analysis gave the upper and lower control limits for an X-chart when the process was in control as 10.5 and 7.7 minutes respectively. What is your estimate of the standard deviation of the waiting time for a customer? Construct a moving-average control chart using a span of 3. What conclusions can you draw from the chart?
TABLE 7-23

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Average Average Average Average Sample Waiting Time Sample Wang Time Sample Waiing Time Sample Waiting Time 8.4 6.5 10.8 9.7 9.0 9.9 10.2 8.3 8.6 9.9 9.4 10.2 12 13 14 15 8.8 10.0 9.5 9.6 8.3 17 19 20 4 9 10 9.6