ENBIS-8 in Athens

21 – 25 September 2008 Abstract submission: 14 March – 11 August 2008

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

23 September 2008, 09:20 – 09:40


Submitted by
Philippe CASTAGLIOLA (1), Gemai CHEN (2), Lingyun ZHANG (2)
(1) Université de Nantes & IRCCyN UMR CNRS 6597, Carquefou, France, (2) University of Calgary, Calgary, Alberta, T2N 1N4, Canada
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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