posted 03 May 2001 04:27 AM               

I'm currently looking into developing a variables sampling plan for a safety critical item were there is no room for accepting non-conforming items/lots of product. Failure to protect the consumer from poor quality items/lots could mean a death situation. The product in question has a lower specification limit, is destructively tested and the aim is to obviously exceed the LSL to a certain degree to provide a safety margin whilst giving a high process capability. I'm contemplating using a single sampling plan for the process average where the process standard deviation is unknown. Reason: Destructive testing is required and therefore an expensive exercise, also samples only provide estimates of the population mean and standard deviation. Another thing, keeping costs to a minimum is of importance whilst still providing protection to the consumer.

Questions:
1) Is this the right approach? If not could someone please advise of an altenative!
2) Would a chain sampling plan be better to this situation where acceptance c=0.
3) One thing I don't understand with these sampling plans is that they specify that if n samples are taken and no non-conforities are found, accept the "LOT". What is considered a lot? If I have a batch of 100 items and I test 1 in that batch, is the 100 a "LOT" size?

Thankyou for taking your valuable time to read and answer this INCREDIBLY long question.

posted 04 May 2001 09:24 AM               

This site http://www.variation.com/techlib/standard.html has some standards for download that may be of use.

Regards,
dWizard

posted 04 May 2001 09:45 AM               

Interesting situation. Not having worked in a safety related industry I do not have a good answer for you. Never the less, I would think you should have more than just a sampling plan in place. Do you have any reliability testing or ongoing life testing in place? I would think you should be monitoring the product reliability to determine how much lies in that unacceptable tail of the population.

posted 07 May 2001 04:24 AM               

Thanks,
Nick

quote:

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Originally posted by Rick Goodson:
Nick,

Interesting situation. Not having worked in a safety related industry I do not have a good answer for you. Never the less, I would think you should have more than just a sampling plan in place. Do you have any reliability testing or ongoing life testing in place? I would think you should be monitoring the product reliability to determine how much lies in that unacceptable tail of the population.

With regard to sampling, remember a sampling plan works on the premise of average outgoing quality levels and alpha and beta risk. AQL levels simply put you in the position of saying I am willing to accept an X probability of accepting product that has Y level of defectives. Take a look at www.samplinplans.com there is a lot of good information there.

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posted 08 May 2001 09:44 AM               

While the chain sampling plan is certainly an option, the OC curve for a c = 0 plan is poor. The fraction defective could double or triple and your probability of acceptance will still be extremely high. If you could use a variables sampling plan you would have the advantage of better protection for the same sample size.