The Generally Weighted Moving Average Variance Chart

Abstract

This article extends the generally weighted moving average (GWMA) technique for detecting changes in process variance. The proposed chart is called the generally weighted moving average variance (GWMAV) chart. Simulation is employed to evaluate the average run length (ARL) characteristics of the GWMAV and EWMA control charts. An extensive comparison of these control charts reveals that the GWMAV chart is more sensitive than the EWMA control charts for detecting small shifts in the variance of a process when the shifts are below 1.35 standard deviations. Additionally, the GWMAV control chart performs little better when the variance shifts are between 1.35 and 1.5 standard deviation, and the 2 charts performs similar when the variance shifts are above 1.5 standard deviation. The design of the GWMAV chart is also discussed.

Keywords:

• ARL
• EWMA
• GWMA
• GWMAV

Mathematics Subject Classification:

• Primary 90B30
• Secondary 62P30

Acknowledgment

The authors thank the referees for their valuable comments and suggestions which greatly enhanced the clarity of this article. All of the suggestions were incorporated directly in the text.