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About this lesson
SPC Corrective Actions
There are appropriate and inappropriate responses that can be taken based upon the data found on a control chart. Actions for special cause variation is different than that for common cause variation.
When to use
The SPC control chart should be updated in real-time with the current process performance. Whenever the chart indicates the need for action, that action should begin immediately.
SPC Control Charts are to be used for process control. That means the information needs to be available to those who are making decisions concerning process operations. In most cases, this means process operators and process managers. In addition, any good control system provides immediate feedback on performance. Based upon these principles there are three attributes that should be associated with using control charts for monitoring and tracking.
- The control charts should be updated in real-time, or as near real-time as is practical. This will allow the operators to take immediate action if a problem is identified.
- The control charts should be maintained where the process operators and managers are located. This normally means that they are located with the process. (Not sequestered in a location that is removed from the process)
- The process operators and process managers must be trained in how to update the control charts and how to read the charts to recognize when a special cause variation condition has occurred.
A common mistake made by those who have not been trained or do not understand SPC, is to treat the control limits as inspections criteria for conforming or non-conforming process results. The control limits are based solely on the voice of the process. The customer spec limits could be wider, narrower, higher or lower than the control limits. The control limits should never be used in that fashion. Rather they indicate process stability, not conformity.
When a process that had been stable and under control shows the indications of special cause variation, the process operator should immediately stop the process to investigate the special cause. In some cases, the process operation may be continued, but only when the special cause condition leads to conforming results. This should not be the normal practice, but only an exception.
When the process is stopped for a special cause, the operator should investigate the process operation to isolate the special cause conditions. Once the special cause is understood, the business can make a decision how best to control that condition and restart the process.
Even though there is no special cause variation present in a process, action may be needed. However, the action will be based upon the conformance to customer spec limits. If the process is stable but creating non-conforming results, the process should be managed so as to center the results in the middle of the customer spec limits. This will take maximum advantage of the available spec limits. It is important to note, that the control chart does not include those spec limits, so this must be done based upon inspection data. Also, when the process is centered, it may violate the special cause rule for a shift of the mean, depending upon the magnitude of the shift. This is an intentional special cause and is not a reason to stop the process and investigate. If the common cause variation is wider than the spec limits, the business should consider changing to a different process with less variation.
Hints & tips
- Some organizations use their SPC charts as a mark of excellence in their management practices and use them for displays. If they are going to be at all useful, they need to be at the workstations and maintained with real-time data.
- Do not include the spec limits on your control charts as an additional threshold line. This leads to confusion on the purpose and use of control charts. Keep inspection separate from process management.
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