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Quality Inspection: Why an AQL Limit of 2.5% Means Nothing

by Renaud Anjoran on 3 Aug 2018

Many Chinese suppliers try to reassure their potential customers by claiming they are certain to pass a QC inspection with an AQL of 2.5%. When asked for documentation that explains precisely what they mean, they usually have nothing to offer.

Let’s say that, as a buyer, you decide to apply the ISO 2859-1 standard (single sample plan based on acceptance sampling), like most other importers. You apply normal severity, level II, as is most often practised.

Let’s look at what could make it easier, or harder, for a manufacturer to pass the final inspection.

 

1. AQL limits can be higher or lower

This one is obvious. If you set an Acceptance Quality Limit (AQL) for major defects at 2.5%, it is much easier to pass the inspection than if the AQL is 0.65%.

AQL_Defect_Table_Example
In this example, the maximum number of ‘major’ defective pieces varies from 3 to 10!

 

2. Defective pieces can be in 1, 2, 3, or 4 categories

Most buyers of consumer goods follow this approach:

  • Zero tolerance for critical defects
  • 2.5% for major defects
  • 4.0% for minor defects

 

You could be stricter this way (as the same number of defective pieces gets added up in only 1 category):

  • Zero tolerance for critical defects
  • 2.5% for major defects
  • No defect is counted as minor (they are actually counted as major).

 

And you could be more lenient this way (the number of defective pieces gets spread over 4 categories):

  • Zero tolerance for safety defects
  • 2.5% for major defects
  • 4.0% for minor defects
  • 6.5% for inconsequential defects

 

3. The definition of defects could be more or less strict

Let’s say you buy apparel, and you sell it in relatively upscale boutiques. Most importers consider a 1cm untrimmed thread as a minor defect, but you classify it as a major defect. That’s a serious difference! If the operators trimming the products don’t know about this, their looser standard might cause a failed inspection.

Define product defects

Another example: let’s say you buy electrical products and you decide to have “the same quality standard as Apple”. You decide that any scratch visible from 30 cm away is unacceptable, and thus is a major defect. But the average Chinese manufacturer will be unable to comply! They might have to re-think the way they process, store, and process the parts.

 

4. You might combine different products into one sampling lot… or do individual sampling

The way quality inspectors find defective pieces is tightly linked with the way they do the sampling (e.g. the number of samples they pull randomly from the total quantity). I provided several examples in the previous article (see quality inspection level and sampling lot).

 

Let’s say you order 2 styles of shirts and 1 style of tie.

You may want the tie to be inspected in a separate lot. This way, if its quality is not acceptable, it will probably be rejected.

AQL_Table_Example_Rejected

 

Or you may decide to bundle it in the same lot as the shirts. The inspection is more likely to pass in this situation, assuming the shirts are fine, even if 10% of the ties are defective.

 

AQL_Table_Example_Accepted

 

5. You could make sure the inspector is better at spotting issues

There are different ways to do this. Here are two examples:

  • Sending an inspector who is particularly good at noticing color differences (about 3% of the population has that gift).
  • Forcing the manufacturer to provide stronger lighting and a dark table.
     

What else could be tweaked? Do you have any other ideas?
Feel free to discuss in the comments section!

 

quality inspection software guide download

 

Topics: AQL

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About the Author

Renaud Anjoran

Renaud has 10 years of experience in the quality and manufacturing fields and is a certified ISO 9001 lead auditor and ASQ certified quality engineer. He was quoted in the New Yorker and the LA Times, and his articles have been published in Quality Progress, Business Insider, and more. His role is to ensure SynControl really solves customer's problems and saves them money.

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