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