A firm of tax advisers is offering a new phone-in service where, for a small fee, customers can get 10 minutes of tax advice over the phone. The firm wants to monitor the length of calls so as not to give customers more time than they have paid for, or to give them less time than they expect. The following table shows samples of six calls taken on different days during a period when the service had 'settled in'.
(a) Calculate the mean, upper control limit and lower control limit for an X and R chart that could be used to monitor calls.
(b) Plot the results for the nine days shown in the table. Do you have any comments about these results?

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Ten-minute tax call advice sampled call lengths in minutes Sampled 12/3 calls 123456 10 17 9 8 12 11 14/3 8 86 12 12 9 17/3 11 10 11 11 10 8 19/3 9 13 10 8 10 8 Date 20/3 26991 10 23/3 10 10 11 9 11 12 24/3 9 9 11 11 10 9 27/3 8 10 8 11 12 11 29/3 9 7 10 13 12 12