Use an Acceptance Xbar-R Chart to detect when the out-of-specification percentage might exceed an acceptable level, and
monitor the variation using subgroup ranges. Use when you have a subgroup size of 8 or
less.
For example, a quality engineer monitors deviations from the target fill weight for a packaged
product. The deviations are measured in tenths of an ounce. The upper specification
limit is 21.3 tenths of an ounce above the target. The process is allowed to shift as
long as no more than 1% of the output is expected to exceed the upper specification
limit. The engineer collects subgroups of 5 packages at regular intervals and creates an
Acceptance Xbar-R Chart.
The R chart does not show evidence of unusual variation, so the engineer can
interpret the Acceptance Xbar chart. Three subgroup means are above the upper acceptance
control limit. These points indicate that the process mean shifted far enough
toward the upper specification limit that more than 1% of the process output might
exceed the specification limit.
Where to find this control chart
To create an Acceptance Xbar-R chart, choose Stat > Control Charts > Acceptance Charts > Xbar-R.
