DJN said:
I have a process that has points out of control. However,the CP figure is 2.29, which I believe is a six sigma process. I understand that modified control limits may be applied in this instance, but have been unable to find any information on the subject. Can anyone help on this subject?
David
Es far as I know, the calculus of Capability or Performance Index have nothing to do with the calculation of Control Limits.
Most of statiscians say that you shouldn´t calculate Cp when is out of control, and some recommend to use performance index in that case (Pp, Ppk).
I tink as I read in some posts "One size doesn´t fit all", that some patterns doesn't apply to some process, if the process is a continuos ones, it's obvious that runs will appear, don't use out of control rules without tinking if they must apply or not.
If you are using IX (individual ond moving range), The autocorrelation could be a problem, I tink the control limits affected by an autocorrelation factor are called "modified control limits ", Don Wheeler show this on "Advanced Topics in Statistical Process Control".
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Sorry, I want to add something, I readed, Modified Control Charts use
Upper Spec - 3 Sigma(X) + 3 Sigma (X_average)
Lower Spec - 3 Sigma(X) + 3 Sigma (X_average)
The try to fuzze the spec limits to obtain action limits, the fuzzed limits are called Reject Control Limits. It is thought that as long as the subgroup averages are within these limits the process will be producing conforming product.
:thedeal: