What is the AQL

What is the “AQL” (Acceptance Quality Limit) in QC?

What Does AQL Mean?

‘AQL’ stands for ‘Acceptance Quality Limit’, and is defined as the “quality level that is the worst tolerable” in ISO 2859-1. It represents the maximum number of defective units, beyond which a batch is rejected. Importers and manufacturers usually set different AQLs for critical, major, and minor defects.

For example: “AQL is 1.5%” means no more than 1.5% defective items in the whole order quantity, on average over several production runs with that supplier.

The “AQL Tables”

The “AQL tables” are statistical tools at the disposal of inspection levelers (for product inspections). They are an industry standard.

They help determine two key elements:

1. How many samples should be picked and inspected, among a batch of product or parts?

2. Where is the limit between acceptability and refusal, when it comes to defective products?

A look at AQL in more detail

The ‘AQL’ is the “quality level that is the worst tolerable” on average over a period covering a number of batches.

In practice, three types of defects are often distinguished. For most consumer goods, the limits are:

Quality Control Tips

 0% for critical defects (totally unacceptable: a user might get harmed, or regulations are not respected).

 2.5% for major defects (these products would usually not be considered acceptable by the end user).

 4.0% for minor defects (there is some departure from specifications, but most users would not mind it).

These proportions vary in function of the product and its market. Components used in building an airplane are subject to much lower AQL limits.

Note that this tool is used mostly during final outgoing inspections (when the products are ready to be shipped out), and sometimes during production (when the number of products is sufficient to have an idea of the batch’s average quality).

Getting familiar with the “AQL tables”

Before using the AQL tables, you should know three parameters:

The ‘lot size’

If ordered different products, consider each product as a separate lot. (The quantity of each product is the lot size). If 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, stick to the so-called “level II” under “normal severity” and to single sampling plans.

The AQL limits

The AQL limits 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

For example, assume the ‘lot size’ is comprised between 3,201 pcs and 10,000 pcs, and that the inspection level is ‘II’. Consequently, the code letter is “L”.

Second table: single sampling plans for level II inspection (normal severity)

In this sample code letter is “L”, so will have to draw 200 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 10 products with major defects AND NO MORE than 14 products with minor defects are found. For example, if you find 15 products with major defects and 12 products with minor defects, the products are refused. If you find 3 with major defects and 7 with minor defects, they are accepted.