The "AQL tables" are statistical tools at the disposal of buyers (for product inspections). They help determine two key elements:
In virtually every production batch, there will be defective products. It is true even after the manufacturer has checked each individual product and has repaired the defective ones.
Thus, in a supplier/buyer relationship, the supplier cannot be expected to deliver defect-free goods. However, the buyer wants to control the quality of purchased goods, since he does not want too many defects. But what does "too many" mean?
How to set the limit between acceptability and refusal in a way that can be agreed upon and measured?
The limit, as described above, is called the ‘AQL’. It stands for ‘Acceptance Quality Limit’, and is defined as the “quality level that is the worst tolerable” (ISO 2859 standard).
For example: “I want no more than 1.5% defective items in the whole order quantity” means the AQL is 1.5%.
In practice, three types of defects are distinguished. For most consumer goods, the limits are:
These proportions vary in function of the product and its market. Components used in building an airplane are subject to much lower AQL limits.
Before using the AQL tables, you should decide on three parameters:
There are basically two tables. The first one tells you which ‘code letter’ to use. Then, the code letter will give you the sample size and the maximum numbers of defects that can be accepted.
First table: sample size code letters
How to read this table? It is very easy.
If you follow my example, I assume your ‘lot size’ is comprised between 3,201pcs and 10,000pcs, and that your inspection level is ‘II’. Consequently, the code letter is “L”.
Second table: single sampling plans for level II inspection (normal severity)
How to read this table?
Your code letter is “L”, so you will have to draw 200pcs randomly from the total lot size.
Besides, I assume you have set your AQL at 2.5% for major defects and 4.0% for minor defects. Therefore, here are the limits: the products are accepted if NO MORE than 10 major defects AND NO MORE than 14 minor defects are found.
For example, if you find 15 major defects and 12 minor defects, the products are refused. If you find 3 major defects and 7 minor defects, they are accepted.
Note: in quality inspections, the number of defects is only one of the criteria. It is sometimes called “quality”, or “quality findings”. The other criteria are usually on the inspector’s checklist, which typically includes:
Source: www.qualityinspection.org (Go there to read more about AQL, its a great site)