Title: A variable sampling interval S2-EWMA control chart for monitoring the process variance

Authors: P. Castagliola, G. Celano, S. Fichera, F. Giuffrida

Addresses: Universite de Nantes & IRCCyN UMR CNRS 6597, Institut Universitaire de Technologie de Nantes, Departement Qualite Logistique Industrielle et Organisation, 2 avenue du Professeur Jean Rouxel, BP 539, 44475 Carquefou Cedex, France. ' Dipartimento di Ingegneria Industriale e Meccanica, University of Catania, Viale Andrea Doria 6, 95125 Catania, Italy. ' Dipartimento di Ingegneria Industriale e Meccanica, University of Catania, Viale Andrea Doria 6, 95125 Catania, Italy. ' Dipartimento di Ingegneria Industriale e Meccanica, University of Catania, Viale Andrea Doria 6, 95125 Catania, Italy

Abstract: This paper proposes a Variable Sampling Interval version of the Fixed Sampling Interval S2-EWMA control chart developed by Castagliola (2004) and dedicated to the monitoring of the sample variance of a process. In this paper, we explain how the various parameters of this VSI S2-EWMA control chart can be computed and how the use of the VSI feature significantly improves the statistical efficiency of FSI S2-EWMA chart, thus representing an effective tool in the detection of process out-of-control conditions. An optimal design strategy based on the Average Time to Signal (ATS) is presented and a comparison with the FSI procedure is performed.

Keywords: sample variance; exponentially weighted moving average; EWMA control charts; logarithmic transformation; adaptive sampling interval; variable sampling interval; process monitoring; SPC; statistical process control; quality control; optimal design.

DOI: 10.1504/IJTM.2007.011807

International Journal of Technology Management, 2007 Vol.37 No.1/2, pp.125 - 146

Published online: 23 Dec 2006 *

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