One‐Sided Modified EWMA Control Charts for Monitoring Time Between Events

Adverse events are frequently encountered in manufacturing and service processes. Control charts are commonly used to monitor adverse events. In high‐quality processes, the adverse events count rate can be very low, with zero counts for multiple time intervals. As a result, control chart limits become narrower, which may increase the number of false alarms. Thus, in high‐quality processes, control charts for monitoring the time between events (TBE) are more effective than control charts for monitoring adverse events. This article presents one‐sided exponentially weighted moving average (EWMA) control charts, based on power transformation and model reset modification, that monitor the downward and upward shifts in the exponentially distributed TBE data. These control charts are called lower‐ and upper‐sided TBE control charts. The designs of lower‐ and upper‐sided TBE control charts are based on a two‐level optimal model that optimizes the parameters of the control charts by minimizing the expected average run length (EARL) and expected weighted run length (EWRL). In addition, comparisons based on average run length (ARL) of the TBE control charts against the existing TBE control charts reveal the superiority of the TBE control charts over the existing TBE control charts. Finally, two real‐life data applications are also provided to highlight the implementation and sensitivity of the TBE control charts, revealing the superiority of the TBE control charts over the existing TBE control charts.