Can I reduce the Sample Size? Sampling check of more than 1,000 pieces daily

Helmut Jilling

Auditor / Consultant
#21
Re: Can I reduce the number of sample size?

One more time. The AQL levels do not "allow for known defect levels to be acceptable". This is not the intent of the standards, nor is it how they are written. Even in "accepted" lots, any defectives found in the sample, or otherwise identified must be removed and dispositioned. To use ANSI/ASQ Z1.4 and Z1.9 correctly, it is necessary to read the document, not just the sampling tables, and use them appropriately. The AQL is an indexing method for sampling plans, which allows one to estimate, when using a sampling plan at a given AQL, how large a percentage defective the process will need to be at before the sampling plan begins rejecting them. These plans assume that the samples are being taken from a stable, ongoing process, and so state in the standard. If the process is not stable or on-going, then these plans are not appropriate. In fact, the plans require the suspending of sampling if too many defectives are found, since the process is no longer stable. The sampling level is an economic decision. If the economic consequences of missing a negative shift in process output is low, and the cost of inspection is high, then a high AQL with an associated low sample size is justified. If the economic cost of the missed shift is high, and the cost of inspection is low, then a low AQL with the associated high sample size is justified. Notice, please that neither of the preceeding two sentences even MENTIONED an allowable defect rate, which in either case could be zero, 3.4 PPM, 10% or whatever to which the contracting parties agree.

To go back to the original poster's question, the best way to negotiate a reduction in AQL, is to show the customer the data. Proceed something like this:

Here is the data that shows our process is stable and capable.
Here is the data on the risk of negative capability excursions.
Here is the data on the cost of the current sampling scheme.
We believe the cost of a missed negative excursion is about X. It will be an estimate, since at least some, if not most of the cost is the customer's.
Based on this data, we believe an AQL of Y, leading to this new sampling scheme, is more appropriate and cost effective.


Geoff Withnell
Geoff, I feel like we are saying a similar thing in a different manner.

In a c=0 plan, if one defect is found the lot must be quarantined and 100% inspected.

In an AQL plan, (going on memory, I no longer have any copies), finding a defect part or two in the sample, with an AQL of 4, the lot would be passed. The two bad parts found would be removed, but the remainder would likely still have more defects, based on the percentage identified in the relevant column. Isn't that how it works? That is deemed acceptable quality, even though there is a high liklihood there may be a few more defects in the lot. That is what I meant by sending defective parts.

I was making the argument that if one gets better results from the process by having higher standards, then the sampling sizes can go down tremendously. Of course, as you suggest, you can try to make an economic argument to a customer that more inspection is not economical. However, more and more customers are rejecting that argument and requiring 0 defects.

The last client I had that still practiced AQL levels, did not understand their customers were not accepting AQL. They were rejecting 50,000 pc lots with 1 or 2 defects.
 
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Geoff Withnell

#22
Re: Can I reduce the number of sample size?

I agree that c=0 plans make more sense. I prefer c=0 plans, and have instituted them whenever I had that power for the very reason you point out. However, even C=0 plans have associated AQLs! Deciding to go to c=0 plans does not eliminate the exercise of determining the appropriate AQL, it just means you will be using the lowest sample size for that AQL, and will not accept a lot if any defectives are found in the sample. Note that it is quite possible that the defective found may be the only defect in the entire lot. I have seen this more than once, where a set-up piece was inadvertently included, or similar "fliers". Too many organizations have sample sizes that are inappropriately large, or small, or which should be doing either no inspection or 100% inspection, because the sample size or AQL was just "pulled out of the air". And a rational c=0 program is not just using the c=0 sample plans from the ANSI standard. You are then side stepping a lot of the calculations involved, and aren't really getting the AQL coverage you think you are. I strongly recommend using "Zero Acceptance Number Sampling Plans, Fourth Edition, Squeglia, Nicholas L."

Geoff Withnell

Geoff, I feel like we are saying a similar thing in a different manner.

In a c=0 plan, if one defect is found the lot must be quarantined and 100% inspected.

In an AQL plan, (going on memory, I no longer have any copies), finding a defect part or two in the sample, with an AQL of 4, the lot would be passed. The two bad parts found would be removed, but the remainder would likely still have more defects, based on the percentage identified in the relevant column. Isn't that how it works? That is deemed acceptable quality, even though there is a high liklihood there may be a few more defects in the lot. That is what I meant by sending defective parts.

I was making the argument that if one gets better results from the process by having higher standards, then the sampling sizes can go down tremendously. Of course, as you suggest, you can try to make an economic argument to a customer that more inspection is not economical. However, more and more customers are rejecting that argument and requiring 0 defects.

The last client I had that still practiced AQL levels, did not understand their customers were not accepting AQL. They were rejecting 50,000 pc lots with 1 or 2 defects.
 
S

spacemanR

#23
YKT,
If you have enough evidence to prove the production process is stable, such as good cpk value and control chart.
I think, your client can accept your requisition on reducing the inspection sample quantity.
 
J

joshua_sx1

#24
Those information given above are quite helpful… you are really great guys!! :rolleyes:

…anyway, I’m not going to add more on “numerical” aspects but on the principle of continual improvement…

…if you can prove that you are improving (using quality tools that are acceptable from your customer), the very next thing to do is to reduce the inspection activities – that’s continual or continuous improvement afterall…
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#25
Re: Can I reduce the number of sample size?

Of course, if I were the Customer I would want to know how you are controlling your process to detect and correct any shifts or drifts in the process. (SPC with REAL reaction plans?) Remember past stability is no guarantee of future stability...
I agree with Bev D on this. In my mind acceptance sampling should be used for incoming receiving, where the product is homogeneous and the goal is to verify that the upstream processes were controlled and generating good product.

For outgoing product, acceptance sampling may be done as a mandate for final inspection to duplicate how the customer will inspect the product. But the key is to control the process.

The worst thing you can do is use acceptance sample sizes to determine line inspection frequency. As the process owner, you have more information to work with, such as time sequence information on process performance that the customer does not have in their pile of parts in a box. I would hope that properly implemented SPC for the key characteristics in question would be an acceptable alternative to acceptance sampling in the light of this. Otherwise, the customer may be overcautious or lack a thorough understanding of process control.

Either way, and in any event, the cost of inspection is a cost, and should be a part of the piece price. If there is evidence that a more cost efficient and equally successful alternative is available - and the benefit can be shared - some customers will think harder on their requirements :cool:
 
T

tata347

#26
Re: Can I reduce the number of sample size?

Is there any statistical documentation for the "skipped lots" inspection? (referenced in Post 11) In some cases the products have had zero failures, mfg yields of 99.9% for years...I need justification to eventually get to a dock to stock scenario but haven't seen anything I can present to the ISO or FDA as sound proof. any advise?

tata
 
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