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 7-28. 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?

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TABLE 7-28 Average Average Average Average Sample Waiting Time Sample Waiting Time Sample Waiting Time Sample Waiting Time 1 8.4 6 9.4 11 8.8 16 9.9 2345 6.5 3 10.8 4 9.7 9.0 10 7890 10.2 12 10.0 17 10.2 8.1 13 9.5 18 8.3 7.4 14 9.6 19 8.6 9.6 15 8.3 20 9.9