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Xbar & S Control Charts are more efficient than Xbar & R Control Charts and should always be used for subgroup sizes greater than five.
They are used as a pair, the Xbar (Mean) chart plots the mean of subgroups that are taken periodically from the process. The S (Standard Deviation) chart plots the standard deviation of the subgroups. See 'Rational Subgroups' for guidance on how to select subgroups.
The control limits are calculated from:
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Upper Control Limit
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Lower Control Limit
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| S Chart |
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| Upper Control Limit |
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| Lower Control Limit |
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The factors are taken from standard tables.
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