Let us start with the formal name and designation: "acceptable quality level", sampling method for inspection by attributes (ISO2859-1:1999).
The AQL is an important metric for companies aiming for a Six Sigma level of quality control.
It makes it possible to assess factually whether a batch in production is acceptable according to defined criteria or must be rejected.
It is widely used both for its statistical significance and for economic reasons.
For example, let us assume that your supplier is to deliver 20000 T-shirts for 20000$ by the end of the month.
In one case you could have someone check 100% of the products:
20000 inspected t-shirts at a rate of 2/min = 20 man-days =5600$.
Your t-shirt went from 1$ to 1,28$
You have 300 T-shirts rejected; your supply manager has no certainty of what this means as a supplier rating; how would he know how much is acceptable? If you have 301 defective T-shirts, should you think about changing the supplier? how about 1000 defective T-shirts?
Now let us try with AQL samping
With a general inspection level of 2 for a batch between 10001 and 35000 and an AQL threshold of 2.5 for major defects and 4.0 for minor defects, we will inspect 315 products.
Max. major defects =14
Max. minor defects = 21
315 T-shirts inspected at a rate of 2/min = 1 man- day = $280.
The cost of your t-shirts is 1.014
The sample size is large enough to give an approximate percentage of defective products
You find 2 major defects and 4 minor defects. Your Supply Manager is satisfied with the quality and has a clear picture of the situation.
He now has an important performance indicator with which he can evaluate the performance of his suppliers and set targets for them.
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