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Why the AQL Inspection System Is Still So Widespread

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Why the AQL Inspection System Is Still So Widespread

In the 1930s, some smart statisticians at Bell Labs worked on an amazing amount of manual calculations to suggest sampling plans for product quality inspections. Over time, it became the topic of MIL STD standards and one of those plans became quite widespread in industry: the AQL inspection system.

AQL now stands for “Acceptance Quality Limit”. Today, acceptance sampling inspections based on that approach are used on too many products to mention: clothing, electronics, toys, homeware, you name it…
And some say that this approach is outdated and less helpful for today’s manufacturers. But if that’s so, why is the AQL system still so widespread?

 

First, what does AQL actually mean?

As I wrote in this popular introduction to AQL:

‘AQL‘ stands for ‘Acceptance Quality Limit‘, and is a method used by many businesses to check a random sample from the production batch of their products and confirm that the risk of bad quality is relatively low.

In ISO 2859-1, the AQL is defined as the “quality level that is the worst tolerable.” 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. Most Asian exporters are familiar with this type of setting.

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, and I accept a certain amount of risk that I make the wrong decision based on the imperfect information coming from checking only a sample of the whole batch”.

 

Can we afford to do AQL inspections today?

Statistically, acceptance sampling inspections will use an AQL limit below which the lot will ‘pass’. Critics of the AQL system feel that this dooms the buyer to receive a surprisingly high proportion of defective products.
If we look at the tables, a part of the response is obvious:
General inspection level II on a sample of 501-1200 pieces, with the usual “2.5 for major and 4.0 for minor defects” limit, gives an acceptance quality limit of 5, and up to 7 defects are accepted in a sample of 80 pieces.
AQL tables - sampling and acceptance limits
If you ask buyers, “do you want 12 defective pieces per 80 of your finished products?”, they may consider that to be too high. And platforms like Amazon will typically penalize stores with too many returns or poor reviews — never mind the rapid damage that bad online reviews can do to a business.
Why is it so favorable to the supplier?
What most people don’t realize, because they haven’t actually read the standard, is that the producer’s risk of having a good batch rejected is set at twice that of the consumer’s risk of having a bad batch accepted.
Shocking? Yes, I know… but that approach was designed for a continuing series of lots from the exact same production process. The manufacturer has the “benefit of the doubt” to a certain extent. That was the intent. Now, if you don’t have that intent, the AQL inspection system is not a great fit for your needs.

 

Not so fast, the AQL inspection system still has a role to play even if it is imperfect

The AQL isn’t as outdated as you may think! It has evolved since WW2. Look at the various editions of MIL-STD 105. For example, a pretty significant change was about the switching rules. The standard kept being perfected. But the original intent hasn’t changed.
Unfortunately, as long as manufacturers (especially those in some Asian countries) do NOT have good pre-production validations and manufacturing process control, there will be a need for acceptance sampling inspections.

 

What options does the buyer have?

  1. Ideally, the buyer would have trust in the manufacturer’s very good track record and would need no inspection. Good pre-production verifications & validations, good QA processes at the source, good process controls in general, good staff training, etc. can help a lot.
  2. As a second choice, an accept-on-zero plan would reduce the number of samples to check and would put a lot of pressure on the supplier. (See my earlier article about the most common sampling plans.)
  3. As a third choice, starting with acceptance sampling based on the LQ would be much more favorable to the buyer, compared to the AQL. And then switching to the AQL is fine once there is a relatively good track record and it really is a continuous production of the same product (since that’s what the AQL is designed for).
    The problem, when the manufacturer is not familiar with the AQL and the LQ concepts, is that starting with the LQ would elicit resistance (once they understand it’s much harsher to them) and confusion. If the inspection fails, they might spend hours just looking for ways to question the methodology. That’s a big problem for the buyer, and quite predictable in certain countries.

 

How realistic is ditching the AQL system for acceptance-on-zero?

We all want zero defects, but for products mass-produced in very large quantities achieving this is not that realistic for a lot of SMEs.
A big company that owns a manufacturing facility, or that has a great influence over a supplier, can dump the acceptance sampling based on the AQL, at least for finished products. They can go in direction of an acceptance-on-zero plan, they can work hand in hand with the people to set up a continuous sampling/screening plan, and so on and so forth because they have the budget and/or influence over the supplier to do so. This is not true for relatively small companies that source products from a distance.
All that to say, acceptance sampling based on the AQL is not great, but there is still a LOT of it going on. Getting rid of it would require a massive educational effort. Unfortunately, I found very, very few people in the QA/QC industry have spent the time to study the topic, so how to expect others to do so?

 

P.S. Read even more posts about the AQL inspection system