Paint primer thickness

Firstly mention that it is important to detect changes in process variability. Standard deviations could be used but it is more common in practice to use the range of successive samples — this is more sensitive to extreme measurements (usually bad but good in the context of quality control).

Also mention that the details of calculating the control limits from the training data are complex but unimportant.

The data are shown in grey (against the grey axis); the sample ranges are shown in black. Drag the slider to assess future samples against the control limits for the range.

There is no indication of a change in the process variability.


The data set contains thickness of paint primer in mils (an imperial measurement equal to one thousandth of an inch), measured from a sample of 10 items each morning and afternoon for 5 successive mornings and afternoons. We regard these data as a training set from which we obtain control limits for later samples of primer thickness.