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When should you recalculate limits?

Eventually, everyone using SPC charts will have to decide whether they should change the control limits or leave them alone. There are no hard and fast rules, but here are some thoughts to help you make your decision.

The purpose of any control chart is to help you understand your process well enough to take the right action. This degree of understanding is only possible when the control limits appropriately reflect the expected behavior of the process. When the control limits no longer represent the expected behavior, you have lost your ability to take the right action. Merely recalculating the control limits, however, is no guarantee that the new limits will properly reflect the expected behavior of the process either.

  1. Have you seen the process change significantly, i.e., is there an assignable cause present?

  2. Do you understand the cause for the change in the process?

  3. Do you have reason to believe that the cause will remain in the process?

  4. Have you observed the changed process long enough to determine if newly-calculated limits will appropriately reflect the behavior of the process?

You should ideally be able to answer yes to all of these questions before recalculating control limits.

To create control charts and easily recalculate control limits, try software products like SQCpack.

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