Use the Display additional σ limits at option on the Limits tab of the Options dialog box to add sigma limits to your control chart. Minitab draws additional limits at the standard deviation levels that you specify.
Usual control limits. No additional limits are drawn.
Additional sigma limits can help you identify shifts and drifts or other patterns in the data.
Use this procedure to add sigma limit lines to your control chart. This procedure demonstrates how to add lines at 1 and 2 standard deviations to an individuals chart. However, you can use the same basic procedure for other control charts and for other standard deviation values.
Suppose that you want to create an individuals chart with red and blue lines at 1 and 2 standard deviations, respectively.
SLIMIT 2 1; CLIMIT; COLOR 25 74 44;
The numbers you list after COLOR specify colors from the outermost to the innermost limits.