Auxiliary information based exponentially weighted moving co-efficient of variation control chart: An application to monitor electric conductivity for water system

Abstract Monitoring the coefficient of variation (CV) is a successful approach in statistical process control to monitor the process variability when the process mean and standard deviation are not constant. In this study, we proposed auxiliary information based exponentially weighted moving CV control chart by using a three-parameter logarithmic transformation to monitor the small and moderate change in process CV. The new chart is compared to the considered charts by means of average run length. The application of the proposed chart is demonstrated to monitor the electric conductivity of a water system by incorporating aggregate hardness of water as an auxiliary variable.