Overview for Acceptance Xbar Chart

Use an Acceptance Xbar Chart to detect when the out-of-specification percentage might exceed an acceptable level.

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 20.6 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 10 packages at regular intervals and creates an Acceptance Xbar Chart.

Two subgroup means during the first upward shift 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.

Note

The process variation must be in control before you assess the process center using an Acceptance Xbar chart because the acceptance control limits are calculated using the within-subgroup variation. If the process variation is not in control, the acceptance control limits are not reliable and Minitab might produce a false out-of-control signal. Use an R chart or S chart to assess whether the process variation is in control.

Where to find this control chart

To create an Acceptance Xbar chart, choose Stat > Control Charts > Acceptance Charts > Xbar.

When to use an alternate control chart

If you do not have subgroups, use Acceptance Individuals Chart.