Quality Control by AQL

What is AQL?

‘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 usually set different AQLs for critical, major, and minor defects.

For example: “AQL is 1.5%” means “I want 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 buyinspection levelers (for product inspections). They are an industry standard. Most suppliers involved in international trade are familiar with it.

They help 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?

AQL Calculator

How AQL calculator is works, at first you should enter data of th batch which you are control:

• Batch Count : If you are controlling different types of products batch count should be entered for specific product seprately

• The inspection level: Different inspection levels will command different numbers of samples to inspect.

• The AQL limits: The AQL limits appropriate for your market. If you accept very few defects, you might want to set a lower AQL for defects product.

The inspection level

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.

How to read this table?
Imagine that your ‘lot size’ is comprised between 3,201 pcs and 10,000 pcs, and that your inspection level is ‘II’. Consequently, the code letter is “L”.

How to read this table?
Your code letter is “L”, so you will have to draw 200 pcs randomly from the total lot size. Besides, Imagine 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.

The 3 “general” inspection levels

Level I

However, settling on this level by default, in order to spend less time/money on inspections, is very risky. The likelihood of finding quality problems is lower than generally recommended.

Level II

It is the most widely used inspection level, to be used by default.

Level III

If a supplier recently had quality problems, this level is appropriate. More samples are inspected, and a batch of products will (most probably) be rejected if it is below the quality criteria defined by the buyer.

Some buyers opt for level-III inspections for high-value products. It can also be interesting for small quantities, where the inspection would take only one day whatever level is chosen.

The 4 “special” inspection levels

These special levels can be applied in cases where only very few samples can be checked. “Four additional special levels, S-1, S-2, S-3 and S-4 […] may be used where relatively small sample sizes are necessary and larger sampling risks can be tolerated” (ISO 2859-1 standard).

Under S-3 level, the number of samples to check is lower than under S-4, and so on.

In practice: for consumer goods, quality control is usually performed under the general levels.

The special levels are used only for certain tests that either take lots of time or destroy the samples. Another situation where special levels are appropriate is a container-loading supervision–to have an idea of what is inside the cartons, without spending too much time at that checking.

AQL Calculator Result

After entering the AQL parameters the result will be shown such as below:

# Number of samples to pick

The inspector(s) will need to pick 200 samples out of the whole batch.

# Maximum number of defectives samples

The inspection is passed if all 3 conditions are met:

• No critical defect is found.
• Defects are found on no more than 14 sample(s). 

Meaning: If you found more than 14 samples defected you should reject whole the batch.

Conclusion 

By this concept you will be able to set up a precise controlling system. And you can setup your system with different paraemeters such as per products, per category, per supplier and so on.

Sepecially in online business this concept will speed up the QC process and will have a direct impact on KPIs and leads to have customer satisfaction.

On the other hand, by this concept making a decision on different types of suppliers is much easier. With proper monitoring system based on historical data you are be able to make a decision which supplier has a best products quality and which one offers lower quality of goods.  

References:

Anjoran, R. (2021, January 28). What is the AQL? QualityInspection.org. https://qualityinspection.org/what-is-the-aql/