Interpreting a Moving Average Sigma Chart

Always look at the Moving Sigma chart first. The control limits on the Moving Average chart are derived from the average moving sigma, so if the Moving Sigma chart is out of control, then the control limits on the Moving Average chart are meaningless.

On the Moving Sigma chart, look for out of control points. If there are any, then the special causes must be eliminated. Remember that the Moving Sigma is the sigma between subgroup averages, so look for process elements that would increase variation between the subgroups in a cell. Brainstorm and conduct Designed Experiments. Note that Auto Drop feature for removing out of control subgroups is not available within our SPC software when analyzing Moving Average charts, since each plotted point contains multiple subgroups.

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

Learn more about the SPC principles and tools for process improvement in Statistical Process Control Demystified (2011, McGraw-Hill) by Paul Keller, in his online SPC Concepts short course (only $39), or his online SPC certification course ($350) or online Green Belt certification course ($499).