Interpreting an EWMA Chart

Standard Case (Non-wandering Mean)

Always look at Range chart first. The control limits on the EWMA chart are derived from the average Range (or Moving Range, if n=1), so if the Range chart is out of control, then the control limits on the EWMA chart are meaningless.

On the Range chart, look for out of control points. If there are any, then the special causes must be eliminated. Remember that the Range is the estimate of the variation within a subgroup, so look for process elements that would increase variation between the data in a subgroup. Brainstorm and conduct Designed Experiments.

After reviewing the Range chart, interpret the points on the EWMA chart relative to the control limits. Run test rules are never applied to a EWMA chart, since the plotted points are inherently dependent, containing common points. Never consider the points on the EWMA chart relative to specifications, since the observations from the process vary much more than the Exponentially Weighted Moving Averages.

If the process shows control relative to the statistical limits for a sufficient period of time, then we can analyze its process capability relative to requirements. Capability is only meaningful when the process is stable, since we cannot predict the outcome of an unstable process.

Wandering Mean Chart

Look for out of control points. These represent a shift in the expected course of the process, relative to its past behavior. The chart is not very sensitive to subtle changes in a drifting process, since it accepts some level of drift as being the nature of the process. Remember that the control limits are based on an exponentially smoothed prediction error for past observations, so the larger the prior drifts, the more insensitive the chart will be to detecting changes in the amount of drift.