...But What Do They Show Us?

Variable charts provide a pair of measures:

Variability should be analysed first. Where the range or standard veviation chart indicates a state of statistical control, the process can be considered stable and predictable.

The mean or median is considered second. If the mean or median chart indicates that the process is stable, then any changes in status over a period of time are probably the effects of common causes.

Variable and Attribute charts are analysed and interpreted in essentially the same way. When properly monitored, they highlight signals of special causes in a process which stand out against the background noise of common cause variation.