Trimmed MeanX̄andRCharts

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Abstract
A modified approach to the computation of control limits for X̄ and R charts is introduced. This procedure consists of replacing X̄ with the trimmed mean of the subgroup averages, and R̄ with the trimmed mean of the subgroup ranges. Standard tables of control limits may continue to be used, requiring only adjustment by a constant multiplier, which is tabulated. The proposed control chart limits are shown to be less influenced by extreme observations than their classical counterparts, and to lead to tighter limits in the presence of out-of-control observations. These concepts are illustrated by several examples.

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