The dispatch operation of a large technology firm receives and records service calls from customers.This dispatch operation is an important interface with the firm's customers, and it is critical that customers' service calls be taken promptly and efficiently. Customers are put on hold when placing their service calls and the length of time customers on hold is of interest.The target value for hold time is 16 seconds.It was anticipated that, due to seasonal variation, there would be fewer incoming calls in the summer months.Due to vacation time, operator staffing is below normal as well at this time.Data were collected from the dispatch operation for 70 days during the months of July to October.Three variables were measured each day:Staff = the number of operators on duty, Calls = the number of incoming calls, and Wait = the total waiting time in hours totaled for all customers. The variable Wait seconds = the total waiting time each day in seconds.Wait seconds = 3600*Wait.The variable Wait seconds/call = the average number of seconds a customer waits per call each day.Wait seconds/call = (Wait seconds)/(Calls).The following is a time series plot of the Wait seconds/call for this period.What does this plot tell you about the wait time of this operation?Is the wait time stable over time?see image:i) This plot contains only common causes, the wait time is stable.ii) The wait time improved from about time 27 to time 49.iii) There is a special cause because of the downward shift in the data.iv) Both ii) and iii) are true.

Time Series Plot of Wait seconds / call 60 - 50 - 40 Wait seconds/ call 30 20- 10- 7 14 21 28 35 42 49 56 63 70 Index