Shewhart Charts for Monitoring the Variance Components

Abstract

The monitoring of process variance is important in ensuring consistent quality. It is more effective and appropriate to monitor the decomposed variance components rather than the overall variance, if the associated special causes of respective variance components can be identified. In the literature, the Shewhart chart with approximate control limits has been proposed for monitoring the variance components, but we show this method is insensitive to detecting increases in the process variance. In this paper, the probability distribution of the sample statistic of variance component is derived as an exact formula, from which the exact Shewhart limits are obtained. Simple procedures for designing Shewhart charts for monitoring variance components are provided. The application of the Shewhart charts is demonstrated with real manufacturing data.

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