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

R

ramvaidhya - 2009

#11
Re: Can I reduce the number of sample size?

I just add you one more way to escape from checking 1000pcs regularly on all lots. There is a provision in Inspection methodology called skip lot system, widely used in Auto segment. For example, just by this if your lots are consistantly accepted for 5 times you can waive from all lot checking for another 3 times then check the 4th one. If fourth is accepted, then again waive 3 lots then continue check on 4th one like that.
 
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Helmut Jilling

Auditor / Consultant
#12
Re: Can I reduce the number of sample size?

I've a question on AQL which I'll need help to decipher...

Let say I'm producing FMCG products, where within 1 day, I'll be producing quite a high number of products. Now, if my client imposed an AQL number, which requires me to do a sampling check of more than 1,000 pieces check per day, is there another way out, whereby I do not need to inspect 1000 pieces at one go, but still satisfying the AQL ?

What if I can prove my Cpk is good ? Can I escape from doing 1000 over pieces of inspection ?

My point is, is there a way of 'by-passing' the required number of samples in the sampling plan, for a given AQL ????
The best way is to improve your processes so you make fewer defects, and so you don't ship defective parts to customers. When you have control over that, AQL ceases to matter. My clients do not ship defective parts, none of them follow any AQL less than C=0 to my knowledge, and I am not aware of any of them that manually inspect more than a few parts (however, many have automatic inline checks).

They have earned the right to do small inspections, based on excellent performance.
 
D

Dave Strouse

#13
Re: Can I reduce the number of sample size?

Helmut,
You wrote

My clients do not ship defective parts, none of them follow any AQL less than C=0 to my knowledge, and I am not aware of any of them that manually inspect more than a few parts (however, many have automatic inline checks).
Are you implying that c=0 plans protect against shipping defective product?
 

Helmut Jilling

Auditor / Consultant
#14
Re: Can I reduce the number of sample size?

Helmut,
You wrote



Are you implying that c=0 plans protect against shipping defective product?
I am stating that c=0 performance ensures against shipping defective product. Plans alone don't do anything except drive you to improve performance.

However, c=0 plans require you to quarantine any sample groups where any defective parts are found. If you do that, it certainly should significantly reduce any defects being shipped.
 
G

Geoff Withnell

#15
Re: Can I reduce the number of sample size?

The best way is to improve your processes so you make fewer defects, and so you don't ship defective parts to customers. When you have control over that, AQL ceases to matter. My clients do not ship defective parts, none of them follow any AQL less than C=0 to my knowledge, and I am not aware of any of them that manually inspect more than a few parts (however, many have automatic inline checks).

They have earned the right to do small inspections, based on excellent performance.
OK there is a significant misunderstanding of what AQL is, and how it should be used going on here. C=0 is not an AQL, and it is possible to have any AQL with a c=0 sampling plan. As Bev D noted, the AQL is the defective percentage that will be ACCEPTED approximately 95% of the time. This does not mean (and ANSI/ASQ Z1.4 is very clear about this) that the ANY defects are acceptable. The decision to do sampling is an economic decision, and the AQL level is that level at which it makes economic sense to sample. The risk of accepting a lot where the defect level has increased significantly (this is what Z1.4 sampling is designed to detect) and the costs associated with that risk need to be weighed against the cost of doing the inspection. The choices are to do 100% inspection, sampling at some level, or no inspection. The choice that makes the most economic sense for you and your customer should be used. Juran's Quality Handbook has, IIRC, a good discussion of how to do the calculation.

Geoff Withnell
 

Helmut Jilling

Auditor / Consultant
#16
Re: Can I reduce the number of sample size?

OK there is a significant misunderstanding of what AQL is, and how it should be used going on here. C=0 is not an AQL, and it is possible to have any AQL with a c=0 sampling plan. As Bev D noted, the AQL is the defective percentage that will be ACCEPTED approximately 95% of the time. This does not mean (and ANSI/ASQ Z1.4 is very clear about this) that the ANY defects are acceptable. The decision to do sampling is an economic decision, and the AQL level is that level at which it makes economic sense to sample. The risk of accepting a lot where the defect level has increased significantly (this is what Z1.4 sampling is designed to detect) and the costs associated with that risk need to be weighed against the cost of doing the inspection. The choices are to do 100% inspection, sampling at some level, or no inspection. The choice that makes the most economic sense for you and your customer should be used. Juran's Quality Handbook has, IIRC, a good discussion of how to do the calculation.

Geoff Withnell
Thank you for the technical clarification. I was using c=0 in a generic sense.

My point is, many customer want zero defects shipped. That is the only acceptable quality level for many customers. This forum is the ONLY palce I hear AQL still tossed around. It is a rather antiquated concept - what defect level is acceptable to most of your customers.

The point was, improve your process robustness, then a sample plan is not an issue.

Thank you, Geoff.
 

Jim Wynne

Staff member
Admin
#17
Re: Can I reduce the number of sample size?

My point is, many customer want zero defects shipped. That is the only acceptable quality level for many customers. This forum is the ONLY palce I hear AQL still tossed around. It is a rather antiquated concept - what defect level is acceptable to most of your customers. The point was, improve your process robustness, then a sample plan is not an issue.
AQL is not an "antiquated concept"; it's a misunderstood concept, which makes it seem antiquated. It's the misunderstanding that's passé. Sooner or later, regardless of "robustness" (or maybe "robustivity"? :tg:) lots will have to be sampled. If you don't know how to rationally determine how to do the sampling, you're probably going to waste time and get unexpected results.
 

Helmut Jilling

Auditor / Consultant
#18
Re: Can I reduce the number of sample size?

AQL is not an "antiquated concept"; it's a misunderstood concept, which makes it seem antiquated. It's the misunderstanding that's passé. Sooner or later, regardless of "robustness" (or maybe "robustivity"? :tg:) lots will have to be sampled. If you don't know how to rationally determine how to do the sampling, you're probably going to waste time and get unexpected results.
Fair point. Sampling plans are not antiquated. Most suppliers still sample.

Perhaps I should have said the notion that there is an "acceptable" level of defects that can be shipped is antiquated. The various AQL levels will allow for known defect levels to be acceptable, and thus shipped, based on major and minor.

I know of no customers in my circles that would find that acceptable. Perhaps because my clients have to achieve those levels of effectiveness, is why they consistently achieve such high performance levels. Anything less is simply not accepted by their customers.

Playing off the OP's question, is there any point to sampling 1000 pcs., but still accepting a certain level of defects?
 

Jim Wynne

Staff member
Admin
#19
Re: Can I reduce the number of sample size?

I know of no customers in my circles that would find that acceptable. Perhaps because my clients have to achieve those levels of effectiveness, is why they consistently achieve such high performance levels. Anything less is simply not accepted by their customers.
I know of many companies who set PPM levels >0 for suppliers. That doesn't mean that the defective units, should they be received, are "acceptable." Generally speaking, suppliers can expect to be debited for defective units, regardless of the prescribed PPM level. PPM levels are generally used as triggers for corrective action, and not to signify that some percentage x of defective product will be accepted.

Playing off the OP's question, is there any point to sampling 1000 pcs., but still accepting a certain level of defects?
Maybe yes, maybe no. If we accept that a sample of ~1000 pieces is actually necessary (and it probably isn't in most instances), sometimes "accepting" (read: accepting reality) a proportion of defective product is the most economical alternative. Granted, those instances should be rare, but they do happen.
 
G

Geoff Withnell

#20
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

Fair point. Sampling plans are not antiquated. Most suppliers still sample.

Perhaps I should have said the notion that there is an "acceptable" level of defects that can be shipped is antiquated. The various AQL levels will allow for known defect levels to be acceptable, and thus shipped, based on major and minor.

I know of no customers in my circles that would find that acceptable. Perhaps because my clients have to achieve those levels of effectiveness, is why they consistently achieve such high performance levels. Anything less is simply not accepted by their customers.

Playing off the OP's question, is there any point to sampling 1000 pcs., but still accepting a certain level of defects?
 
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