Differential Smoothing in the Bivariate Exponentially Weighted Moving Average Chart

The multivariate exponentially weighted moving average (MEWMA) control chart proposed by Lowry et al. (1992) has become one of the most widely used charts to monitor multivariate processes. Its simplicity, combined with its high sensitivity to small and moderate process mean jumps, is at the core of its appeal. Lowry et al. (1992) advocated equal smoothing of each quality variable unless there is an a priori reason to weight quality …