Chart Preparation Exercise (3)

As well as calculating the control limits, you should also understand the criteria which are used to create an appropriate scale for each of the graphs. They are:

• It should cover the range of probable values
• It should provide space above and below the control limits for plotting out-of-control values.
• When the possible values may be above or below a nominal or target value, the nominal should be roughly the middle of the scale.

Looking at your printed chart, you should hopefully agree that the scales used on both of the two graphs meet these criteria.