A quality engineer monitors deviations from the target fill weight for a packaged product. The deviations are measured in tenths of an ounce. The engineer has determined that the process variation is stable. 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 process 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.
- Open the sample data, FillWeights.MWX.
- From the drop-down list, select All observations for a chart are in one column and enter Fill weight deviation.
- In Subgroup sizes, enter 10.
- In Upper spec, enter 20.6.
- In Accepted percentage outside of each spec limit, enter 1.
- Select OK.
Interpret the results
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.
The other shifts in the process mean are within the acceptance control limit and do not indicate that the accepted percentage is exceeded. The engineer should investigate the two out-of-control subgroups to determine whether the shift reflects an additional cause or a stronger effect from an existing cause. Possible causes include a machine adjustment, a change in raw material, an operator change, or a different production lot.
