Re: Can I reduce the number of sample size?
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.
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
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
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.