From: "Saved by Internet Explorer 11" Subject: Back to basics: What is the AQL? Date: Wed, 9 Nov 2016 08:59:37 -0800 MIME-Version: 1.0 Content-Type: multipart/related; type="text/html"; boundary="----=_NextPart_000_0063_01D23A67.992BE940" X-MimeOLE: Produced By Microsoft MimeOLE V6.1.7601.17609 This is a multi-part message in MIME format. ------=_NextPart_000_0063_01D23A67.992BE940 Content-Type: text/html; charset="utf-8" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.globalsources.com/NEWS/SIC-Back-to-basics-what-is-the-aql.html =EF=BB=BF
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Back=20 to basics: What is the AQL?=09 =20 =20Posted: January = 30,=20 2012 |
by Renaud Anjoran
The "AQL tables" are statistical tools at the disposal of buyers (for = product=20 inspections). They help determine two key elements:
The need for an objective measurement of quality
In virtually every production batch, there will be defective = products. It is=20 true even after the manufacturer has checked each individual product and = has=20 repaired the defective ones.
Thus, in a supplier/buyer relationship, the supplier cannot be = expected to=20 deliver defect-free goods. However, the buyer wants to control the = quality of=20 purchased goods, since he does not want too many defects. But what does = "too=20 many" mean?
How to set the limit between acceptability and refusal in a way that = can be=20 agreed upon and measured?
Definition and application of 'AQL'
The limit, as described above, is called the 'AQL'. It stands for = 'Acceptance=20 Quality Limit', and is defined as the "quality level that is the worst=20 tolerable" (ISO 2859 standard).
For example: "I want no more than 1.5% defective items in the whole = order=20 quantity" means the AQL is 1.5%.
In practice, three types of defects are distinguished. For most = consumer=20 goods, the limits are:
These proportions vary in function of the product and its market. = Components=20 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:
There are basically two tables. The first one tells you which 'code = letter'=20 to use. Then, the code letter will give you the sample size and the = maximum=20 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=20 3,201pcs and 10,000pcs, and that your inspection level is 'II'. = Consequently,=20 the code letter is "L".
Second table: single sampling plans for level II inspection = (normal=20 severity)
How to read this table?
Your code letter is "L", so you will have to draw 200pcs randomly = from the=20 total lot size.
Besides, I assume you have set your AQL at 2.5% for major defects and = 4.0%=20 for minor defects. Therefore, here are the limits: the products are = accepted if=20 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=20 are refused. If you find 3 major defects and 7 minor defects, they are=20 accepted.
Note: in quality=20 inspections, the number of defects is only one of the criteria. It is = sometimes=20 called "quality", or "quality findings". The other criteria are usually = on the=20 inspector's checklist, which typically includes:
Frequently asked questions about AQL
"What are the reduced and tightened inspection=20 severities?"
They are designed to be used in very specific situations, when a = producer is=20 particularly reliable, or on the contrary fails too often. In practice, = these=20 severities are used in less than 1% of QC inspections.
The normal severity already allows for a good variation of sample = sizes. In=20 the vast majority of cases, third-party inspectors follow only the = normal=20 severity.
"So, basically I have to authorize the factory to produce = some=20 defects?"
Yes, some defects, since a factory cannot reasonably be expected to = turn out=20 100% good quality.
However, it does not mean the buyer tolerates everything as long as = the=20 number of defects are below the AQL limits. Please see below the note = issued in=20 the ISO2859 standard:
"Although individual lots with quality as bad as the acceptance = quality limit=20 may be accepted with fairly high probability, the designation of an = acceptance=20 quality limit does not suggest that this is a desirable quality level. = Sampling=20 schemes [...] are designed to encourage suppliers to have process = averages=20 consistently better than the AQL."
"Based on my AQL, I calculated the = proportion of defects authorized. Why don't they correspond to the = maximum=20 number of defects authorized?"
It is true. In our example above, 2.5% of 200 samples is 5 samples, = but we=20 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=20 simple, the producer runs a risk of rejection (based on the random = element when=20 drawing the sample) even though his products (if they were all checked) = would be=20 accepted. And, in the same logic, the consumer runs a risk of accepting = bad=20 products. The statisticians had to account for these risks, that's why = the=20 numbers were adjusted and seem not to make sense.
"Why not just say, 'we'll check 10% of the quantity', or = whatever=20 percentage deemed appropriate?"
Here again, the statisticians tell us it is not that simple. As we go = up in=20 the total quantity, the proportion of products checked can decrease, for = the=20 same confidence in the inspection results.
As you can see in the chart below, the number of samples to check = (vertical=20 axis) increases at a slower pace than the total quantity (horizontal = axis).
Renaud Anjoran is the founder of Sofeast Quality Control and helps = importers=20 to improve and secure their product quality in China. He writes advice = for=20 importers on the Quality = Inspection=20 blog. He lives full time in Shenzhen, China. You can contact him at = =20 This e-mail address is being protected from spambots. You need = JavaScript=20 enabled to view it =20 .