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Quality Quandaries: How to Set Up a Robust Shewhart Control Chart for Dispersion?

Summary: In this article, the authors have outlined procedures to construct Phase I and Phase II standard deviation control charts. The initial estimate of the population standard deviation for the Phase I standard deviation control chart is based on the 10% trimmed mean of the sample interquartile ranges. This estimator is robust against both diffuse and localized disturbances so that the Phase I limits are not affected by these disturbances. The Phase I data are then screened for localized and diffuse disturbances by means of both a Phase I sample standard deviation control chart and a Phase I individuals chart. At the end of Phase I, the population standard deviation is estimated from the screened data using the mean of the sample standard deviations, ensuring efficiency under normality. The procedures are described step by step and give constants that can be plugged into the formulas directly, thus making them easy for practitioners to implement and apply.

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  • Topics: Quality Tools
  • Keywords: Shewhart control chart, Control charts, Robust design, Dispersion, Standard deviation, Estimation, Screening
  • Author: Nazir, Hafiz Z.; Schoonhoven, Marit; Riaz, Muhammad; Does, Ronald J.M.M.;
  • Journal: Quality Engineering