The EWMA chart for the standard deviation is a useful tool for monitoring the variability of a process quality characteristic. The performance of this chart is usually evaluated under the assumption of known parameters. However, in practice, process parameters are estimated from an in-control Phase I data set. A modified EWMA control chart is proposed for… (More)
In the present article, we study the effect of estimating the vector of means and variance-covariance matrix in the performance of the MEWMA control chart. We show through simulation that the performance of the MEWMA control chart is affected when the parameters are estimated compared to the known parameters case. The control limit that achieves the desired… (More)
It is quite common in industry to resort to grouped observations especially in cases where registering the exact value of the characteristic of interest is difficult or costly. In this paper we present a new methodology for handling grouped data arising from processes involving more than one correlated variables.