We have an acceptance sampling inspection in place where we use the ANSI/ASQ Z1.4 -2013 standard under Normal Inspection, using General Inspection Level II to drive our samples size and accept, reject criteria. We do not uses switching rules as we have always found them too difficult to manage. I have two questions.
If I have one lot that fails acceptance sampling and I am trying to bound the issue is it suitable to bound it to the one affected lot if the lot before and after pass or do I need to carry out additional sampling.
My second question is if I have a batch that passes acceptance sampling but at a subsequent downstream process a defect being inspected for by the upstream acceptance sampling inspection is found how do I determine if the lot is acceptable? Do I trust the acceptance sampling inspection or react?
The first question is not an uncommon one and actually it is a good practice to isolate the lot and do 100% inspection of it. That way you can estimate the % defective and if another failure occurs in the next 5 lots, then increase the sampling until you have some confidence that the supplier has fixed the problem. Once that confidence is restored, then you go back to what you inspected originally.
The second question, is one that you have to understand how well do you follow the acceptance sampling process? If your alpha level is at 95%, 5% of the time, you can accept a bad batch as good. That is the pure definition of the alpha risk. If this failure falls within the 5%, your process is working and while you sort through the lot, and notify the supplier, it is not something that you over react to.
I hope this helps.
James Bossert, PhD, MBB, CQA, CQE, CqM/OE
Sr Performance Improvement Consultant
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