A Johnson’s type transformation EWMA S2 control chart

This article demonstrates how a four parameters (bounded) Johnson SB transformation combined with an EWMA approach can be used to monitor the sample variance of a process. The resulting chart will be called an EWMA-J-S2 chart. In this article, the computation of both the control limits and the parameters of the Johnson SB transformation of the EWMA-J-S2 chart are explained. An easy-to-use table providing useful constants and an illustrative example are given. The distributional properties of the transformed sample variance are derived and, finally, an optimal design strategy based on the ARL is presented and a list of optimal design parameters for the EWMA-J-S2 chart is provided.