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Path: Sourcing
News & Advice >> Import
from China Advice >> Back to basics: What is the AQL?

Back
to basics: What is the AQL?

Posted: January 30,
2012

by Renaud Anjoran

The "AQL tables" are statistical tools at the disposal of buyers (for product
inspections). They help determine two key elements:

How many samples should be inspected?

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

The need for an objective measurement of quality

In virtually every production batch, there will be defective products. It is
true even after the manufacturer has checked each individual product and has
repaired the defective ones.

Thus, in a supplier/buyer relationship, the supplier cannot be expected to
deliver defect-free goods. However, the buyer wants to control the quality of
purchased goods, since he does not want too many defects. But what does "too
many" mean?

How to set the limit between acceptability and refusal in a way that can be
agreed upon and measured?

Definition and application of 'AQL'

The limit, as described above, is called the 'AQL'. It stands for 'Acceptance
Quality Limit', and is defined as the "quality level that is the worst
tolerable" (ISO 2859 standard).

For example: "I want no more than 1.5% defective items in the whole order
quantity" means the AQL is 1.5%.

In practice, three types 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).

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.

Getting familiar with the AQL tables

Before using the AQL tables, you should decide on three parameters:

The 'lot size'. If you ordered different products, the quantity of each
product is a lot size, and separate inspections should be carried out for each
lot. If you ordered only one product, the lot size is your total order
quantity.

The inspection level. Different inspection levels will command different
number of samples to inspect. In this article, we will stick to the so-called
"level II", under "normal severity".

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

How to read this table? It is very easy.

If you follow my example, I assume your 'lot size' is comprised between
3,201pcs and 10,000pcs, and that your inspection level is 'II'. Consequently,
the code letter is "L".

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

How to read this table?

Your code letter is "L", so you will have to draw 200pcs 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 major defects AND NO MORE than 14 minor defects are found.

For example, if you find 15 major defects and 12 minor defects, the products
are refused. If you find 3 major defects and 7 minor defects, they are
accepted.

Note: in quality
inspections, the number of defects is only one of the criteria. It is sometimes
called "quality", or "quality findings". The other criteria are usually on the
inspector's checklist, which typically includes:

Packaging conformity (barcodes, inner packing, cartons, shipping
marks…).

Product conformity (aspect, workmanship…). If all the products are in red
color instead of orange, there is no need to count each sample as a defect. It
makes more sense to refuse for product conformity.

Specific tests defined in the inspection protocol (they might not be
performed on all samples).

Frequently asked questions about AQL

"What are the reduced and tightened inspection
severities?"

They are designed to be used in very specific situations, when a producer is
particularly reliable, or on the contrary fails too often. In practice, these
severities are used in less than 1% of QC inspections.

The normal severity already allows for a good variation of sample sizes. In
the vast majority of cases, third-party inspectors follow only the normal
severity.

"So, basically I have to authorize the factory to produce some
defects?"

Yes, some defects, since a factory cannot reasonably be expected to turn out
100% good quality.

However, it does not mean the buyer tolerates everything as long as the
number of defects are below the AQL limits. Please see below the note issued in
the ISO2859 standard:

"Although individual lots with quality as bad as the acceptance quality limit
may be accepted with fairly high probability, the designation of an acceptance
quality limit does not suggest that this is a desirable quality level. Sampling
schemes [...] are designed to encourage suppliers to have process averages
consistently better than the AQL."

"Based on my AQL, I calculated the
proportion of defects authorized. Why don't they correspond to the maximum
number of defects authorized?"

It is true. In our example above, 2.5% of 200 samples is 5 samples, but we
accept the goods even if 10 samples are found with a major defect.

Why this difference? There are heavy statistics behind this issue. To make it
simple, the producer runs a risk of rejection (based on the random element when
drawing the sample) even though his products (if they were all checked) would be
accepted. And, in the same logic, the consumer runs a risk of accepting bad
products. The statisticians had to account for these risks, that's why the
numbers were adjusted and seem not to make sense.

"Why not just say, 'we'll check 10% of the quantity', or whatever
percentage deemed appropriate?"

Here again, the statisticians tell us it is not that simple. As we go up in
the total quantity, the proportion of products checked can decrease, for the
same confidence in the inspection results.

As you can see in the chart below, the number of samples to check (vertical
axis) increases at a slower pace than the total quantity (horizontal axis).

Renaud Anjoran is the founder of Sofeast Quality Control and helps importers
to improve and secure their product quality in China. He writes advice for
importers on the Quality Inspection
blog. He lives full time in Shenzhen, China. You can contact him at

This e-mail address is being protected from spambots. You need JavaScript
enabled to view it

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