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 the graphs on a control chart. They are:

• It should cover the range of probable values
• It should provide space above and, where appropriate, 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.
• When the range of possible values extends in one direction only, above or below 0, then the scale is usually anchored at 0.

Looking at your printed chart, you should hopefully agree that the current scale for the graph meets these criteria.