Paint primer thickness

Firstly mention the reasons for taking samples rather than individual measurements and producing control charts for sample means:

The control limits are initially for a run chart of individual values. Drag slider to show the samples from the next 15 half-days.

Now click Show means to display the sample means and their control limits:

 ±  3

From both control charts, we conclude that there is no evidence of a shift in the process mean.


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.

(In practice, there are usually more training samples, but we use a small real data set for illustration.)