AQL (Acceptable Quality Level) for Zero Defects

M

mjfqd

#1
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?
 
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Jim Wynne

Staff member
Admin
#2
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?
You should choose an AQL that reflects the risk you're willing to accept. BTW, the second bullet point above makes no sense; all sampling plans "accept on zero defects."
 

bobdoering

Stop X-bar/R Madness!!
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#3
Re: AQL for Zero Defects

I assume you are using the AQL for "lot acceptance", as in incoming receiving or final inspection?
 
M

mjfqd

#4
Re: AQL for Zero Defects

The zero defects statement is listed on the contract. Although the spec says that the AQL should be specified on the contract, the zero defects note is the only clue.

This is for final inspection. We need to show the customer inspector how we develop our sampling plan. I think I'm missing part of the puzzle.
 

Ronen E

Problem Solver
Staff member
Moderator
#5
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 :lol:

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.
 

Jim Wynne

Staff member
Admin
#6
Re: AQL for Zero Defects

The first and third bullets are straightforward.
On the second, I think the intent is to "guarantee" zero defects in production yields.
Linked with the requirement for a sampling plan, I think the second bullet was intended to require C=0 for lot acceptance, and probably has nothing to do with production yield.
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.
The AQL, in lot acceptance sampling, is what determines the sample size. If the AQL were .0001, every lot would have to be inspected 100%, thus no sampling plan would be necessary.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#7
Re: AQL for Zero Defects

On the second, I think the intent is to "guarantee" zero defects in production yields.
In my mind AQL has nothing to do with production yield - only lot acceptance. Capability and throughput are related to production yield. AQL should not be used in relationship to production, anyway. It only works for homogeneous lots either incoming or final inspection.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#9
Re: AQL for Zero Defects

the lot doesn't have to be homogeneous - the sampling from the lot has to be random.
If it is not homogeneous, then it has to be random and large enough to capture the variation. If not homogeneous - may require a larger sample than the standard plans to be effective. Lot definition can be a contributor to the problem (e.g. several streams combined at a batch process that become a lot may be an issue to detect its true variation with insufficient sampling.)

Sampling is so fraught with error that if it is not homogeneous and normally distributed you are likely to get unreliable results.
 

Jim Wynne

Staff member
Admin
#10
Re: AQL for Zero Defects

If it is not homogeneous, then it has to be random and large enough to capture the variation. If not homogeneous - may require a larger sample than the standard plans to be effective. Lot definition can be a contributor to the problem (e.g. several streams combined at a batch process that become a lot may be an issue to detect its true variation with insufficient sampling.)

Sampling is so fraught with error that if it is not homogeneous and normally distributed you are likely to get unreliable results.
 
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