Journal Published Online: 22 June 2018
Volume 47, Issue 1

Optimal Designs of EWMA Charts for Monitoring the Coefficient of Variation Based on Median Run Length and Expected Median Run Length

CODEN: JTEVAB

Abstract

The shape of run-length distribution changes with process shifts. This leads to complexity in interpreting the average run length performance. In this article, we show that the percentiles of the run-length distribution, especially the median run length (MRL), are more intuitive. The 5th and 95th percentiles of the run-length distribution are also provided in order to investigate the variation and spread of the run length. We develop two new optimal-design procedures for the exponentially weighted moving average (EWMA) charts, for monitoring the coefficient-of-variation (CV) squared (EWMA-γ2). These include minimization of the out-of-control MRL and the out-of-control expected MRL for deterministic and unknown shift sizes, respectively. Both the zero and steady states are discussed in this article. The optimal EWMA-γ2 chart is illustrated with real industrial data obtained from a metal sintering process. A comparative study reveals the superiority of the EWMA-γ2 charts for certain ranges of shifts in the CV.

Author Information

Teoh, W. L.
School of Mathematical and Computer Sciences, Heriot-Watt University Malaysia, Putrajaya, Malaysia
Lim, J. Y.
Department of Physical and Mathematical Science, Universiti Tunku Abdul Rahman, Kampar, Perak, Malaysia
Khoo, Michael B. C.
School of Mathematical Sciences, Universiti Sains Malaysia, Penang, Malaysia
Chong, Z. L.
Department of Physical and Mathematical Science, Universiti Tunku Abdul Rahman, Kampar, Perak, Malaysia
Yeong, W. C.
Department of Operations and Management Information Systems, Universiti Malaya, Kuala Lumpur, Malaysia
Pages: 21
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Stock #: JTE20170118
ISSN: 0090-3973
DOI: 10.1520/JTE20170118