Modeling and analysis of EWMA control schemes with variance-adjusted control limits

A stochastic model of the EWMA control chart employing the variance-adjusted control limits is presented. The average run length for various shifts in the process mean is approximated by using a Markov chain approach. Following this, an alternative basis for the design of EWMA schemes is proposed. Computational results are presented and the impact of employing the variance-adjusted control limits on the average run length is investigated. Finally, conditions that permit the use of the asymptotic control limits in place of the variance-adjusted control limits are also identified.