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Shewhart Charts for Monitoring the Variance Components

Summary: [This abstract is based on the authors' abstract.] When the special causes of the variance components can be identified in a monitoring process, it is more effective to monitor the variance components separately than as a whole. The Shewhart chart with approximate control limits has been proposed for this purpose, but is shown to be insensitive to increases in the process variance. It is demonstrated that when the probability distribution of the sample statistic of variance component is derived as an exact formula, exact Shewhart limits may be obtained. Procedures for designing Shewhart charts for variance component monitoring are described and demonstrated with real manufacturing data.

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  • Topics: Quality Tools, Process Management, Quality Control
  • Keywords: Average run length (ARL),Chi-square statistics,Process variability,Variance components,Statistical quality control (SQC),Shewhart control chart,Random effects
  • Author: Chang, T.C.; Gan, F.F.
  • Journal: Journal of Quality Technology