Re: AQL for Zero Defects
I have the following requirements:
- Sampling for inspection shall be IAW ANSI/ASQ Z1.4-2008.
- A sampling plan that accepts on zero defects is required.
- The standard sampling plan shall be Inspection Level II
I have minor and major characteristics defined in the drawing. The AQL is not specified in the contract. How do I determine the AQL to use for the sample size?
Hi,
I'm willing to give it a try
To me, it's quite obvious that whoever wrotw this set of requirements (and is negotiating them with you) has little undretsanding of practical statistics. Therefore, it calls for a try to understand
the intent rather than to literally interpret the text (which is safe but will probably not take you very far).
The first and third bullets are straightforward.
On the second, I think the intent is to "guarantee" zero defects in production yields. Of course we all know that in the real world this is close to impossible, however, you can surely come close enough. If you assure a low-enough probability for a defect (say 10^-6, or whatever you consider appropriate), it can be practically considered as "zero defects". If you are only allowing a 10^-6 defect rate, then your AQL=0.0001.
As to "minor" and "major" indications in drawings, I would assume (and verify with the customer, if possible) that the above relates only to the "major". Lots of companies use the term "minor" for requirements that are not too imporatnt / critical to the product function / safety, and allow much higher AQLs for them. If you don't get a clear indication from the customer for the "minor" AQL level, set a value that makes sense to you, and document the justification (I would say something between AQL=1 and AQL=0.01, but that's truely just a blind guess since I don't know a thing about the product / customer / settings etc.).
Cheers,
Ronen.