A study was done at a large hospital to examine the waiting time in minutes from booking an operation until the patient’s arrival in the operating room. Management considered it important that the waiting time not exceed 12 hours (720 minutes). Data was gathered for 47 consecutive bookings in September 1989. a) The following is a time series plot of this data. Are there any special causes in this data? What action should be taken as a result of this plot?

See image:

i) There is a special cause at time 21. Find out what is different about the conditions at time 21 compared to the other times.

ii) There is a special cause at time 21. Remove this value from the data and replot the time series plot.

iii) There is a special cause at time 21. No follow up is required because the special cause occurred too far in the past to find the reason.