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X-Bar- and R-Chart Control Limits Based on A Small Number of Subgroups

Summary: A common rule of thumb is that conventional control limits for X-bar and R charts, no standards given, should be based on at least 25 subgroups. This article presents information for evaluating the lack of reliability of these control limits when they are based on a smaller number of subgroups. Then, another method is presented for setting the control limits for these charts so that they can be reliably used regardless of how few subgroups have been inspected.

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  • Topics:
  • Keywords: Statistical tests,Control limits,Control charts
  • Author: Hillier, Frederick S.
  • Journal: Journal of Quality Technology