In recent years, the coefficient of variation (CV) chart is receiving increasing attention in quality control. A number of studies demonstrated that adaptive charts could detect process shifts faster than traditional charts. This paper proposes an EWMA chart with variable sampling interval (VSI) to monitor the CV. Formulas for computing the performance measures of the VSI EWMA-γ² chart are derived using Markov chain, where γ² denotes the CV squared. Comparative studies show that the VSI EWMA-γ² chart significantly outperforms other competing charts. An example using real manufacturing data shows that the VSI EWMA-γ² chart performs well in applications.

Journal of the Operational Research Society (JORS) is the world's longest established OR journal and an official journal of The OR Society. It is the aim of the Journal to present papers which are relevant to practitioners, researchers, teachers, students and consumers of operational research, and which cover the theory, practice, history or methodology of OR.

Duncan considered the design parameters of Shewhart control charts in 1956 based on an economic view-point, and ensured that an economic design control chart actually lowers the cost compared with a Shewhart control chart. His research method has been widely used in subsequent studies on the subject and focused on a fixed-sampling interval (FSI). However, it has been discovered that variable-sampling-interval (VSI) control charts are substantially quicker than FSI control charts in detecting shifts in the process. In an earlier study, an economic design of a VSI control chart with a single assignable cause for a continuous flow has been proposed. This research extends a single assignable cause to multiple-assignable causes to construct an economic model of VSI control charts. A numerical example is provided to demonstrate the effectiveness of the proposed model. The result indicates that VSI is more effective than FSI control charts.