On the performance of two-sided EWMA charts in the monitoring of continuous proportions

Abstract

In this work, we study the performance of two-sided EWMA charts for monitoring continuous proportions in Phase II, when individual observations are available. The term continuous proportion refers to an observation in the interval $(0,1).$ There are several models that can be used to describe data of this type (and the respective processes, as well) such as the Beta distribution, the Simplex distribution, and the Unit Gamma distribution. For each of these three models, we investigate aspects of the statistical design as well as the performance of the respective two-sided EWMA charts. Also, apart from providing the appropriate values for the design parameters of each chart, we investigate their robustness when they are applied on the monitoring of processes that follow a different model than the assumed one. Finally, two illustrative examples are also given.

Keywords:

• Beta distribution
• continuous proportions
• EWMA
• robustness
• simplex distribution
• statistical process control
• unit gamma distribution

Acknowledgments

The authors would like to thank the two anonymous reviewers for their constructive comments which improved the content and the presentation of this paper.

Disclosure statement

No potential conflict of interest was reported by the author(s).