ENBIS: European Network for Business and Industrial Statistics

On t and EWMA t Charts for Monitoring Changes in the Process Mean

Abstract

Submitted by

Philippe CASTAGLIOLA

Authors

Philippe CASTAGLIOLA (1), Gemai CHEN (2), Lingyun ZHANG (2)

Affiliation

(1) Université de Nantes & IRCCyN UMR CNRS 6597, Carquefou, France, (2) University of Calgary, Calgary, Alberta, T2N 1N4, Canada

Abstract

The performance of X-bar chart is usually and naturally studied under the assumption that the process variance is well estimated and does not change. This is, of course, not always the case in practice. We find that in the No case, X-bar charts are not robust against errors in variance estimation or changing variance. In this paper we discuss the use of a t-chart and an Exponentially Weighted Moving Average (EWMA) t-chart to monitor the process mean. We show that the EWMA t-chart has the desired robustness property. We also propose new control limits for both the t-chart and the EWMA t-chart.

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