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:
The factors are taken from standard tables.