Application of 'AQL Inspection

Definition and application of 'AQL'

It stands for 'Acceptance Quality Limit', and is defined as the "quality level that is the worst tolerable"

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. 

The need for sampling, rather than 100% checking

 

When controlling the quality of a batch of products, it is not practical to inspect 100% of them (unless the quantity is very small). Inspecting a large number of products takes a long time: it is expensive, and inspectors are less effective as they get tired. Actually, a 100% check does not yield that much more information than inspecting a statistically representative sample.

Why different inspection levels?

 

There is a fairly obvious principle in statistical quality control: the greater the order quantity, the higher the number of samples to check.

But should the number of samples ONLY depend on the order quantity? What if this factory had many quality problems recently, and you suspect there are many defects? In this case, you might want more products to be checked. 

On the other hand, if an inspection requires tests that end up in product destruction, shouldn't the sample size be drastically reduced? And if the quality issues are always present on all the products of a given batch (for reasons inherent to processes at work), why not check only a few samples?

For these reasons, different levels are proposed by MIL-STD 105 E (the widely recognized standard for statistical quality control).

It is usually the buyer's responsibility to choose the inspection level--more samples to check means more chances to reject bad products when they are bad, but it also means more days (and dollars) spent in inspection.

The 3 general inspection levels

Level I

Has this supplier passed most previous inspections? Do you feel confident in their products quality? Instead of doing no quality control, buyers can check less samples by opting for a level-I inspection.

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 prefer level-III inspections for high-value products. It can also be interesting for small quantities, where the inspection would take only one day whatever the level 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."

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.

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? 

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 normal (level II) inspection 

 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.

 

Two examples to get clearer idea

 

Let's say we have ordered 5,000pcs of a product. In the table below, we can see how many samples would be drawn under each of the 3 inspection levels.

General inspection levels
Reduced (I)	Normal (II)	Tightened (III)
80pcs	200pcs	315pcs

As we can see, the numbers of samples to check vary from 80pcs to 315pcs. But a trained inspector might be able to do it in one day, whatever the inspection level you choose.

Now let's say we have ordered 40,000pcs of a product. Again, we can see the differences in sample sizes.

General inspection levels
Reduced (I)	Normal (II)	Tightened (III)
200pcs	500pcs	800pcs

In this case, the inspection might take one day of work (Under level I), two days (under level II), or three days (under level III).