Highlight on AQL Sampling
Quite frequently our clients do ask us the same questions as below:
- What is the most commonly used international standard for product inspections in China and Asia?
- How to determine the sampling size in quality control?
- What’s the acceptable quality inspection level of whole order quantity?
Inspection is the tool which is used to evaluate the conformance of goods to agreed standards or requirements. Inspection is essential and give a reference about the acceptable quality level and margin of error you are ready to allow. Usually quality control are performed only on a representative sample. Indeed, proceed to 100% inspection is costly and time-consuming. As it, AQL (or Acceptable Quality Level), inspection was designed to calculate the number of relevant pieces of a lot to represent the whole, in order to save your time and money while assuring you the maximum quality rate.
AQL is one of the most frequently used standards of quality inspection in customer goods export from China or Asia. Industrials, such as toys, garments, food, electronic products manufacturer… apply this term quite often. As most of decisions regarding shipments for export market are made on AQL sampling plans, it is essential to know what is AQL.
In this article we are going to present the fundamental concept of AQL and how to apply it in the quality inspection in China and Asia.
The ‘Acceptable Quality Limit’ is called the ‘AQL’. It is defined as the “quality level that is the worst tolerable” (source: ISO 2859-1 standard).
The “AQL tables” are statistical manners on the process of product inspections (such as during production inspection, Pre-shipment inspection). They assist to determine two key issues:
- How many samples should be selected and checked, among a batch of cargoes or parts?
- Where is the limit between acceptability and refusal, when it judges to be the defective products?
Application of ‘AQL’ in quality inspection
For example: “We don’t want more than 1.0% defective goods in the whole order quantity, on average over several products run with that vendor” means the AQL is 1.0%.
In practice, three kinds of defects are distinguished. For most consumer goods, the limits are:
- 0% for critical defects (totally unacceptable: a user might get harmed, or regulations are not respected).
- 5% for major defects (these products would usually not be considered acceptable by the end user).
- 0% for minor defects (there is some departure from specifications, but most users would not mind it).
These proportions are different in function of the products and its market. For example: components used in assembling an aircraft are much less influenced by AQL limits.
AQL is applied mostly to pre-shipment inspection (PSI), when the products are ready to be shipped out, and during production inspection (DPI), when the quantity of products is sufficient to have an idea of the cargo’s overall quality.
AQL tables for product inspections
Before using the AQL tables, it’s important to understand three parameters as below:
- The ‘lot size’.
If you order different products, the quantity of each product is a lot size, and it is suggested to perform separate inspections for each lot. If you purchase only one product, the lot size is the total batch quantity.
- The inspection level.
Different AQL levels will lead different quantity of samples to inspect. Here we discuss about “level II” under “normal severity” and single sampling plans.
- The AQL level suitable for your market.
If you only accept very few defects, you might set a lower AQL for both major and minor defects.
There are two basic tables in AQL chart. Table 1 indicates you which ‘code letter’ to use.
For example, if your ‘lot size’ is comprised between 3,201 pcs and 10,000 pcs, and that you choose your inspection level is ‘II’. Consequently, the code letter is “L”.
Table 1: sample size code letters
Once you get the code letter, it will lead you to the table 2 which tells the sample size and the maximum numbers of defects that can be accepted.
Your code letter is “L”, so you will have to randomly pick 200 pieces from the total lot size.
Besides, we assume you have chosen your AQL at 2.5% for major defects and 4.0% for minor defects. Accordingly, the limits are found: the products are accepted if NO MORE than 10 products with major defects AND NO MORE than 14 products with minor defects.
For instance, if you find 18 products with major defects and 10 products with minor defects, the products are rejected. If you find 6 with major defects and 8 with minor defects, they are accepted.
Table 2: single sampling plans for level II inspection (normal severity)
Remark: in quality inspections, the number of defective goods is only one of the criteria. It usually called “quality”, or “quality findings”. The other criteria are often on the inspector’s checklist, which typically includes:
- Packaging conformity (barcodes, inner packing, master cartons, shipping marks…).
- Product conformity (general aspect, pantone color, weight, size measurement, workmanship…). If during the inspection, all the products are in green color instead of blue, there is no need to count each sample as a defect. It makes more sense to reject for product conformity.
- Label & marking (logo, tag, printing…)
- Specific tests in the inspection report (those might not be performed on all inspected samples if they are time-consuming or destructive).
More info about the AQL sampling and inspection
To understand the concept of the Acceptance Quality Limit (AQL) in depth and how do we perform quality inspection, you should read this article.