The acceptable quality limit (AQL) is the worst tolerable process average (mean) in percentage or ratio that is still considered acceptable; that is, it is at an acceptable quality level. In a quality control procedure, a process is said to be at an acceptable quality level if the appropriate statistic used to construct a control chart does not fall outside the bounds of the acceptable quality limits.
The usage of the abbreviation AQL for the term "acceptable quality limit" has recently been changed in the standards issued by at least one national standards organization (ANSI/ASQ) to relate to the term "acceptance quality level". It is unclear whether this interpretation will be brought into general usage, but the underlying meaning remains the same.
AQL helps determine two key elements:
- How many samples should be picked and inspected, among a batch of product or parts?
- Where is the limit between acceptability and refusal, when it comes to defective products?
An acceptable quality level is a test and/or inspection standard that prescribes the range of the number of defective components that is considered acceptable when random sampling those components during an inspection. The defects found during an electronic or electrical test, or during a physical (mechanical) inspection, are sometimes classified into three levels: critical, major and minor.
0 for critical defects Critical defects are those that render the product unsafe or hazardous for the end user or that contravene mandatory regulations. 2.5 for major defects Major defects can result in the product's failure, reducing its marketability, usability or saleability. 4.0 for minor defects Minor defects do not affect the product's marketability or usability, but represent workmanship defects that make the product fall short of defined quality standards.
Different companies maintain different interpretations of each defect type. In order to avoid argument, buyers and sellers agree on an AQL standard, chosen according to the level of risk each party assumes, which they use as a reference during pre-shipment inspection.
How to read the AQL tables
You should know three attributes before using the AQL tables.
- The “lot size”,If you ordered different products, the quantity of each product is a lot size, and it is advised to perform separate inspections for each lot. If you ordered only one product, the lot size is the total batch quantity.
- The inspection level. Different inspection levels will command different numbers of samples to inspect. In this article, we will stick to the so-called “level II” under “normal severity” and to single sampling plans.
- The AQL level appropriate for your market. If your customers accept very few defects, you might want to set a lower AQL for both major and minor defects.
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
If you follow my example, I assume your ‘lot size’ is comprised between 10,001 pcs and 35,000 pcs, and that your inspection level is ‘II’. Consequently, the code letter is “M”.Second table: single sampling plans for level II inspection (normal severity)
Your code letter is “M”, so you will have to draw 315 pcs 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 14 products with major defects AND NO MORE than 21 products with minor defects are found.
For example, if you find 18 products with major defects and 19 products with minor defects, the products are refused. If you find 5 with major defects and 9 with minor defects, they are accepted.