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When
to Use an X-bar / S Chart
X-bar & Sigma charts
are used when you can rationally collect measurements in groups (subgroups).
Each subgroup represents a "snapshot" of the process at a given
point in time. The charts' x-axes are time based, so that the charts show
a history of the process. For this reason, you must have data that is time-ordered,
that is, entered in the sequence from which it was generated. If this is
not the case, then trends or shifts in the process may not be detected,
but instead attributed to random (common cause) variation.
For subgroup sizes greater
than ten, always use X-bar / Sigma charts, since the range statistic is
a poor estimator of process sigma for large subgroups. In fact, the subgroup
sigma is ALWAYS a better estimate of subgroup variation than subgroup range.
The popularity of the Range chart is only due to its ease of calculation,
dating to its use before the advent of computers. For subgroup sizes equal
to one, an Individual-X / Moving Range chart can be used, as well as EWMA
or Cusum charts.
X-bar Charts are efficient
at detecting relatively large shifts in the process average, typically shifts
of +-1.5 sigma or larger. The larger the subgroup, the more sensitive the
chart will be to shifts, providing a rational
subgroup can be formed. For more sensitivity to smaller process shifts,
use an EWMA or Cusum chart.
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