Chart Preparation Exercise (3)

Before starting the calculations, we should consider the scales on each of the two graphs. When deciding on an appropriate scale, the following points should be borne in mind:

• There should not be too many, or too few intervals between the control limits. In practice, there are generally around 10-12.
• The scale should be convenient to use, i.e. it should be easy to plot each point accurately, and without any ambiguity
• There should be space above and below the control limits to plot any out-of-control points.
• When the possible values may be above or below a nominal or target value, the nominal should be roughly in the middle of the scale.
• When the nominal is 0, and the possible values can only be greater than the nominal (as in the case of the range graph), then the scale should be anchored at 0.

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