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
The diagram below shows the ranges of successive paint primer samples, along with control limits that were obtained from the initial five training samples. The details of the calculations for the control limits are complex but unimportant..
The actual data values have also been drawn on the chart in grey to help you see the connection between the sample ranges and the raw data. They are not usually included in range charts.
Use the slider to display the ranges of the samples from the following 15 half-days. The ranges are all within the control limits, so there is no indication of a change in the process variability.
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.