![]() It is assumed that the first occurrence of a point not falling within the predicted limits shows that the system must be unstable since it has changed from the predictive model. This is because any point taken should fall within the statistical predictions. Future data taken to determine process stability can be of any size.The upper and lower control limits for the process can then be determined from this data. The average range is simply the average of subset ranges. The average standard deviation is simply the average of subset standard deviations. ![]() The grand average is the average of all subset averages.
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