From: Gabriel
Date: 07 Aug 2003
Time: 17:29:55
Stan, I have read about all the previous messages on this subject, and found you to be pretty contrary to them.
As I see it, in today's qualty levels achived (typically measured in hundreds of PPM or less and Cpk>1.33), acceptance sampling plans are just useless.
A sampling plan with an AQL of, lets say 500 PPM (or AQL=0.05) is impracticable most of the times, because I need to inspect about 2000 parts to get, on average, one defective, and with such a sample size the consumer risk is still unacceptable.
Now, If I want zero defects (or about that) from my customer, which sampling plan would I use? None. I would expect him to have capable and reliable processes.
But if I still want to do some incoming inspection, at least to monitor that everything is as expected, wouldn't C=0 be the right choice? Remember I don't care about producer's risk because I expect zero defects, and there is no risk to reject a lot if it contains no defective. Isn't the follwing true: - For any given sample size and RQL, C=0 gives the smaller consumer risk. - For any given RQL and consumer risk, C=0 gives the smaller sample size. - Further more, C=0 plans can be allways treated as "sequential". Stop sampling when you find the first defect (and reject the lot) or when you reach n (and accept the lot).
We use samll samples C=0 sampling plans more as a monitoring than as an acceptance plan (both for incoming and final inspection). It is true that the power to detect unacceptable lots is very poor, but any other criteria than C=0 would be worse for the same sample size. Since our quality is based on process control and not on these inspections, using other C would create a vision in our employees that we accept defects, and that would be critical when our quality is based ot the control our wmployees have on the processes thy run. We don't want them to accept defects.
I would like very much your view on this,
Thanks and regards.