Detecting changes to process variability
A control chart of sample means is used to detect shifts in the 'centre' of a process. In a similar way, a control chart to assess whether the process variability has changed can be based on the spread of successive samples.
A control chart for changes in process variability can be based on the sample standard deviations of successive samples, but it is more common in practice to use a control chart based on the sample ranges.
Paint primer thickness
By separately targetting the process centre and variability with a control chart for means and a control chart for ranges, we can get better indications of any changes in the performance of the process, and we can therefore intervene more promptly to correct potential drops in quality.