Add standard deviation lines to your control chart

Examples of additional sigma limits

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.

No additional sigma limits

Usual control limits. No additional limits are drawn.

Additional sigma limits at 1 and 2 standard deviations

Additional sigma limits can help you identify shifts and drifts or other patterns in the data.

For example, to help you notice changes in the process sooner, you can display additional limits at ± 1 and ± 2 standard deviations. Values that you enter affect only the line position for display, not the test results.

Add sigma limit lines to your control chart

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.

  1. Choose Stat > Control Charts > Variables Charts for Individuals > Individuals.
  2. Complete the dialog box as usual.
  3. Click I Chart Options and then click the Limits tab.
  4. In These multiples of the standard deviation, type 1 2 to add lines at 1 and 2 standard deviations.
  5. Click OK in each dialog box.

Add sigma limit lines of different colors

Suppose that you want to create an individuals chart with red and blue lines at 1 and 2 standard deviations, respectively.

  1. Choose Stat > Control Charts > Variables Charts for Individuals > Individuals.
  2. Complete the dialog box as usual.
  3. Click I Chart Options and then click the Limits tab.
  4. In These multiples of the standard deviation, type 1 2 to add lines at 1 and 2 standard deviations.
  5. Click OK in each dialog box.
  6. Choose Editor > Copy Command Language.
  7. Choose Edit > Command Line Editor.
  8. Right-click and choose Paste, or press Ctrl+V.
  9. Under the IChart command, add the following 3 lines:
    SLIMIT 2 1;
    CLIMIT;
    COLOR 25 74 44;

    The numbers you list after COLOR specify colors from the outermost to the innermost limits.

  10. Click Submit Commands.
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