ASQ Z.1 sampling plan

[Mike S.]

You've been given a lot of good information in a short thread.

Statistical sampling is all about risk, who's taking it and how much.

 

[phxsun2001]

Yes, yes that is exactly what I am saying. Let’s take a look at what sampling plans mean.
Any sampling plan must specify either an AQL, RQL or both otherwise you aren’t sampling you are doing 100% inspection. That’s just math.

The AQL is the ACCEPTABLE Quality Level. Let that sink in for a minute…what that means without any equivocation is that a 1% AQL sampling plan will ACCEPT a lot that has 1% defects in it 95% of the time (if there are in fact 1% defects in the lot). It is ACCEPTABLE. So if the sample doesn’t exceed the c=n number of defects then the lot is ACCEPTABLE and it doesn’t need to be screened. Now generally accepted practice is to reject the defective parts and scrap them or not return them to the lot without rework.

Statistical sampling isn’t easy or simple.

Any other questions?

I agree that if the company makes cabin furniture, seats or non critical items, it is acceptable to use Z1.4 and accept lots with know defects. It is not acceptable if the company makes critical parts for aerospace industries. II explained right from the beginning that this company makes critical components for major aerospace companies. If this company knowingly accepts lots with known defects without screening and enter them in their inventory for production, it violates section 8.7 of the AS9100D standard. Their QMS allows them to do it. The lot that did not pass the sampling plan should be segregated, identified and actions taken. Part of the action could be RTV or screen 100%.

8.7.1 The organization shall ensure that outputs that do not conform to their requirements are identified and controlled to
prevent their unintended use or delivery.
NOTE: The term “nonconforming outputs” includes nonconforming product or service generated internally,
received from an external provider, or identified by a customer.
The organization shall take appropriate action based on the nature of the nonconformity and its effect on the conformity of
products and services.

 

[phxsun2001]

Their requirement is in Procedure XYZ stating that ASQ Z1.4 will be used and AQL 1.0 is used on all products inspected.
I am not the customer they are making the products. I was one of four AS9100 auditors during a 5-day recertification audit.

There are three main requirements:
1) AS9100D 8.5.1C2
Ensuring that when sampling is used as a means of product acceptance, the sampling plan is justified on
the basis of recognized statistical principles and appropriate for use (i.e., matching the sampling plan to
the criticality of the product and to the process capability).

There are two requirements in 8.5.1.C2
A) Sampling plan used has to be based on recognized statistical principle and appropriated for use.
Most commonly used plan meet this requirement. Whether it is appropriate depends on criticality of the product among other criteria.
It is a general requirement. It is up to the company to interpret and come up with their sampling procedures.

2) The 2nd requirement come from the company after they interpret the AS9100D standard and come up with their sampling procedures in the form of OP or WI. AS Auditor audit mostly to this requirement.

3) The 3rd requirement comes from interested parties. One example is from their customers. It may be in the Statement of Work or general Quality requirement or Quality clauses in the contract. I did ask about that, but could not get any answer. They have more than 8 key customers and it will take too much time to review the requirements. Customer may specify sampling plans to be used for their products.

I want to make it clear that the products from this company is critical. Failure could cause system or subsystems not getting signals to activate intended functions. If I have to issue a NCR, it would be vs their internal sampling plan procedure. Their Management established a sampling procedure that allows their Receiving Inspection staffs to accept lots with known defects found without screening the lots. It is a violation of 8.7.1. It would have been acceptable if this company manufacture cabin furniture, entertainment units... that are not critical. Again, their products are critical.

Some of the companies I audited use the same ASQ Z1.4 sampling plan, but add a note requiring the lot be rejected and screened 100%, if only one defect is found. Basically they are modifying this attribute plan to be a Zero acceptance plan. They could have just use ASQ H1331 and pick an appropriate AQL.

 

[Bev D]

I don’t disagree that a traditional attributes based sampling plan for safety critical characteristics is not appropriate. But this is from a quality engineering standpoint, not an external auditing standpoint. Also since I am very knowledgable and experienced in statistics I know that c=0 plans make people feel good but they do not preclude the shipment of defects in the lot. (Briefly, the sample sizes are generally smaller than c>0 plans and are still based on ACCEPTABLE defect rates so it is unlikely that all but the most defective lot will be rejected. By much greater I mean substantially greater than the AQL - which you have not called out; 1% is egregiously large for critical defects. A sampling scheme that uses continuous data and SPC is the best approach but will not preclude the creation and shipment of ‘error’ based defects (true attributes data). Only 100% inspection and poke yoke type gauges can do that. So even if you are appalled at the sampling plan the company uses your solution is no better. Which is why I am opposed to a nonconformance. You’re going after the wrong thing. You really should have a better understanding of sampling plans and how they work before you go out and critique others…

As an auditor you have willfully mis-interpreted the requirements you quoted. The Company did what those requirements stated. They decided what sampling procedure to use and they followed it. As you said “it is up to the Company to determine this”. It is not up to a 3rd party auditor. Unfortunately the ASQ/ANSI/Mil-Std plans are generally accepted sample plans.

A Customer can dictate a sampling procedure - but you said it would be too much work to determine this. (For who? You or them?). Too bad. You now have no grounds for a non-conformance.

Perhaps one of the experienced auditors here can provide better advice for how you can issue your nonconformance.

I will add for those who are interested, a c=0 plan that can be fairly effective is one that is based on the RQL (Rejectable Quality Level, not the AQL), uses the straightforward binomial or even Poisson formulas (the difference will be about 1 sample unit) and no regard to lot size. Because the precision of sampling is not based on lot size…). This results in large sample sizes - but we cannot have low risk and low sample sizes.

 

[Mike S.]

... we cannot have low risk and low sample sizes.

A ton of wisdom in 9 words.

OP: You're not the customer. For all you know, the customer is 100% screening these items, making sampling by the supplier low-risk. You're all hung-up on issuing a CAR here and I'm not sure what your motivation is.

 

[phxsun2001]

First of all, I did not issue an NCR to my client during this audit. I discussed it with their management, they thought it was OK because they have been doing it this way for years and no auditor ever raised the issue.

Also, my client will not screen lots with known defects because the Z1.4 plan that they use allows them to accept lots with known defects.

The motivation: I have been conducting AS9100 registration audits for 18 years. When I come across an issue, I go to Elsmar Cove to search for answers. This time I did not see a lot of discussion on this topic. I post it up to generate inputs and feedbacks. That's why I entered the history of changes on this requirement in the last 20 years. Some Auditors may not know that it was a requirement to use some type of zero acceptance plan in 2004 and then it was removed in Rev C because it may not be applicable to non critical products.

Even if I want to issue a NCR, it is not easy to write it up. I can only say that their internal procedure allows Receiving Inspection to accept lots with known defects and it is violating section 8.7.1. Would it be nice if most of the issues we come across are tools not calibrated, shelf-life expired, ...... The most difficult NCR I issued was vs the CMMI software system of one of the top aerospace company in the world. My Registrar backed me up.

 

[Bev D]

and what did you learn from the responses here?

 

[Mike S.]

First of all, I did not issue an NCR to my client during this audit. I discussed it with their management, they thought it was OK because they have been doing it this way for years and no auditor ever raised the issue.

Also, my client will not screen lots with known defects because the Z1.4 plan that they use allows them to accept lots with known defects.

The motivation: I have been conducting AS9100 registration audits for 18 years. When I come across an issue, I go to Elsmar Cove to search for answers. This time I did not see a lot of discussion on this topic. I post it up to generate inputs and feedbacks. That's why I entered the history of changes on this requirement in the last 20 years. Some Auditors may not know that it was a requirement to use some type of zero acceptance plan in 2004 and then it was removed in Rev C because it may not be applicable to non critical products.

Even if I want to issue a NCR, it is not easy to write it up. I can only say that their internal procedure allows Receiving Inspection to accept lots with known defects and it is violating section 8.7.1. Would it be nice if most of the issues we come across are tools not calibrated, shelf-life expired, ...... The most difficult NCR I issued was vs the CMMI software system of one of the top aerospace company in the world. My Registrar backed me up.

I said, "for all you know the customer is 100% screening..." CUSTOMER - as in the customer receiving the product from the supplier you audited. Many times things are sampled at incoming but undergo 100% testing down the line so that any defects are eventually caught. This can be important, because as Bev said, sampling can be risky even if C=0.

I've been "in quality" for longer than 18 years, and I worked under the AS9100 revision requiring C=0 sampling.

Despite what you say you have given zero evidence that they are "violating section 8.7.1".