The effect of parameter estimation on (X)over-bar control charts for the lognormal distribution

Statistical process control (SPC) is a method of monitoring, controlling and improving a process through statistical analysis. In real applications, many industrial process data may follow a positively-skewed distribution such as the lognormal distribution. In this article, under lognormal process, we study the performance of three (X) over bar control charts, Shewhart control chart, weighted variance control chart and weighted standard deviation control chart, based on the in-control expected average run length (AARL). The standard deviation of ARL (SDARL) metric is used to evaluate the performance for various amounts of phase I data. A formula to approximate the ARL by the normal approximation method is established in this study. The in-control and out-of-control performance of the three (X) over bar control charts are compared based on the normal approximation and simulation.