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Control Limits for Group Charts

Summary: [This abstract is based on the authors' abstract.] An application of group charts to a multiple filling head bottling machine at the BYU Creamery motivated an investigation of in-control average run length (ARL). It is shown that ARL of an in-control process with k streams is approximately 370/k. This highlights two properties. First, the group chart in-control ARL is different from the traditional Shewhart chart in-control ARL of 370. Second, the group chart will yield more frequent out-of-control signals, many of which are false alarms. Tables of constants are provided to modify existing group chart limits. The modification results in corrected group charts with constant ARL for any given number of streams.

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  • Topics: Statistical Process Control (SPC)
  • Keywords: Average run length (ARL),Statistical process control (SPC),Statistical methods,Control charts
  • Author: Grimshaw, S. D.; Bryce, G. R.; Meade, D. J.
  • Journal: Quality Engineering