The Elsmar Cove Business Standards Discussion Forums More Free Files Forum Discussion Thread Post Attachments Listing Elsmar Cove Discussion Forums Main Page
Welcome to what was The Original Cayman Cove Forums!
This thread is carried over and continued in the Current Elsmar Cove Forums

Search the Elsmar Cove!

Wooden Line
This is a "Frozen" Legacy Forum.
Most links on this page do NOT work.
Discussions since 2001 are HERE

Owl Line
The New Elsmar Cove Forums   The New Elsmar Cove Forums
  Statistical Techniques and 6 Sigma
  Sampling Question - Long Story

Post New Topic  Post A Reply
profile | register | preferences | faq | search

UBBFriend: Email This Page to Someone! next newest topic | next oldest topic
Author Topic:   Sampling Question - Long Story
thebigbear
Lurker (<10 Posts)

Posts: 4
From:Melbourne Australia
Registered: Jan 2001

posted 03 May 2001 04:27 AM     Click Here to See the Profile for thebigbear   Click Here to Email thebigbear     Edit/Delete Message   Reply w/Quote
Hi everyone,
I need some help clarifying a few things and unfortunately this is a long story but it's necessary to help with answering.

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.

Nick

IP: Logged

dWizard
Forum Contributor

Posts: 20
From:Normandy, TN 37360
Registered: Mar 2001

posted 04 May 2001 09:24 AM     Click Here to See the Profile for dWizard   Click Here to Email dWizard     Edit/Delete Message   Reply w/Quote
This site http://www.variation.com/library.html has some good stuff on sampling.

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

Regards,
dWizard

------------------
I was better but I got over it.

IP: Logged

Rick Goodson
Forum Wizard

Posts: 102
From:Wuakesha, Wisconsin, USA
Registered: Aug 2000

posted 04 May 2001 09:45 AM     Click Here to See the Profile for Rick Goodson   Click Here to Email Rick Goodson     Edit/Delete Message   Reply w/Quote
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.

IP: Logged

thebigbear
Lurker (<10 Posts)

Posts: 4
From:Melbourne Australia
Registered: Jan 2001

posted 07 May 2001 04:24 AM     Click Here to See the Profile for thebigbear   Click Here to Email thebigbear     Edit/Delete Message   Reply w/Quote
Hi Rick,
thankyou for the prompt reply. Yes we do do additional reliability testing on the products and all is fine in that respect. I'm still not sure if I'm on the right track but I've been doing a few calc's and modelling at home and I have a reasonbly good understanding of the construction of OC Curves for sampling plans and the effects of the different parameters. One of a few reasons that made me ask the original set of questions is, "If we are testing a product that is yielding a process capability in excess of 2 and the estimated proportion non-conforming is in the order of 0.000196%, then why sample at the current level?" If all we are getting as a fraction non-conforming is 0.00196%, the probability of acceptance from the OC Curve is going to be exceptionally high. Is this a good case for reducing sample size/frequency? I'm in the opinion that it is and feel that a Chain Sampling plan (Chsp-1)would suit our situation. What is your thought on this, or anyone elses for that matter?

Thanks,
Nick

quote:
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.


IP: Logged

Rick Goodson
Forum Wizard

Posts: 102
From:Wuakesha, Wisconsin, USA
Registered: Aug 2000

posted 08 May 2001 09:44 AM     Click Here to See the Profile for Rick Goodson   Click Here to Email Rick Goodson     Edit/Delete Message   Reply w/Quote
Nick,

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.

Rick

IP: Logged

All times are Eastern Standard Time (USA)

next newest topic | next oldest topic

Administrative Options: Close Topic | Archive/Move | Delete Topic
Post New Topic  Post A Reply Hop to:

Contact Us | The Elsmar Cove Home Page

Your Input Into These Forums Is Appreciated! Thanks!


Main Site Search
Y'All Come Back Now, Ya Hear?
Powered by FreeBSD!Made With A Mac!Powered by Apache!