When do I recalculate control limits?
There is the tendency to recalculate control limits whenever a change is made to the process. However, you should extend the existing control limits out over the new data until you see evidence that the change has had an impact on the data, such as shifting or out-of-control evidence. Then recalculate limits using only data subgroups collected after the change was made.
There are no hard and fast rules for recalculating control limits, but here are some thoughts to help you decide.
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 possible only 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 When the control limits no longer represent the expected behavior, you have lost your ability to take the right action.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 is no guarantee that the new limits will properly reflect the expected behavior of the process.
- Have you seen the process change significantly, i.e., is there an assignable cause present?
- Do you understand the cause for the change in the process?
- Do you have reason to believe that the cause will remain in the process?
- Have you observed the changed process long enough to determine if newly-calculated limits will appropriately reflect the behavior of the process?
Ideally, you should 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.