Improved Phase I Control Charts for Monitoring Times Between Events

In many situations, the times between certain events are observed and monitored instead of the number of events particularly when the events occur rarely. In this case, it is common to assume that the times between events follow an exponential distribution. Control charts are one of the main tools of statistical process control and monitoring. Control charts are used in phase I to assist operating personnel in bringing the process into a state of statistical control. In this paper, phase I control charts are considered for the observations from an exponential distribution with an unknown mean. A simulation study is carried out to compare the in-control robustness and out-of-control performance of the proposed chart. It is seen that the proposed charts are considerably more in-control robust than two competing charts and have comparable out-ofcontrol properties. Copyright © 2014 John Wiley & Sons, Ltd.