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Modified R Charts for Improved Performance

Summary: [This abstract is based on the authors' abstract.] The performance of traditional R charts is analyzed and modifications are introduced for monitoring both increases and decreases in process dispersion. It is shown that the use of equal tail probability limits and some run rules does not significantly improve monitoring ability. It is recommended that the R chart with a central line equal to the median of the distribution of the range be used, and that this chart be supplemented with a run rule that signals when nine consecutive points lie on the same side of the median line. These modifications result in R charts with improved performance for monitoring the process dispersion.

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  • Topics: Process Management
  • Keywords: R chart (range chart), Run length, Run distribution, Average run length (ARL), Run rules
  • Author: Acosta-Mejia, Cesar A.; Pignatiello Jr., Joseph J.
  • Journal: Quality Engineering