A few years ago I wrote about the inspection levels, but I regularly notice that some people are confused about the “special” levels (S1, S2, S3, and S4).
Here is what the ISO 2859-1 standard says (emphasis is mine):
The inspection level designates the relative amount of inspection. Three inspection levels, I, II and III, are given in Table 1 for general use. Unless otherwise specified, level II shall be used. Level I may be used when less discrimination is needed or level III when greater discrimination is required. Four additional special levels, S-1, S-2, S-3 and S-4 are also given in Table 1 and may be used where relatively small sample sizes are necessary and larger sampling risks can be tolerated.
When to use special inspection levels?
The above excerpt from the standard is pretty clear. There are 2 good reasons:
a) The general levels (I, II, III) point to a number of samples that is not realistic
Here are a few examples where these special levels come in handy:
- You want to check the packaging in full, but only on a few cartons.
- You want to perform some time-consuming tests (e.g. a full function test on an electrical product), and you don’t have time to run these tests on all the samples you inspect visually (in level II, usually).
- You want to perform destructive tests (e.g. inspecting bullets by firing them), and obviously you don’t want to destroy too many samples.
b) Checking much fewer samples doesn’t mean the risk of non-detection is much lower
Let’s take a simple example. The casing of an electronic product is made by plastic injection molding.
All of those pieces come out of the same mold. The most important is to validate that the first articles are acceptable and sign off on them. There is still a chance that things go downhill over time, but dimensions will tend to be similar, and many buyers decide to reduce the sampling size on all pieces coming out of a mold. That tradeoff makes sense.
How to read the AQL tables when using special inspection levels?
I prepared this simple ‘whiteboard’ video to explain how it works:
Let’s walk through an example.
Let’s say you are buying 50,000 salt & pepper sets in ceramic.
There are several color combinations, some booklets are included, etc. which means many ways the packing can go wrong. So you want to check the packing in great detail, but only on an S3 level.
In the table below, you see that the code letter is G.
Then let’s go to the other table, and you can find this information:
- You should check the packing on 32 samples.
- If you don’t want to accept a defective packing on more than 1.5% of the quantity, the limit is 1.
- In other words, you accept the batch (as far as packing conformity is concerned) if you find 0 or 1 defective cartons. And you reject it if you find 2 or more.
Note that, when it comes to tests on products and these tests are conducted on a special level, they are considered “critical” (i.e. they are failed as soon as 1 sample fails).
It makes things more simple and avoids lengthy discussions with Chinese suppliers who don’t fully understand how the statistics work.
I hope this explanation was clear. Questions are welcome, so leave a comment in the comments section if you have one!
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Three examples of points checked on different levels
Let’s say you are working on a batch of LED lights. Here are the three checks that may take place on different levels.
A visual check would typically be done on a general level such as II.
Color temperature test
A color temperature test, if very important, might get done on S-2.
An internal check is typically done on S-1, or just on 1 piece (irrespective of the whole batch quantity).
(For a more in-depth overview of the checkpoints that apply on LED products, you can watch this video: LED Products Quality Inspection & Testing Checklist .)
Editor’s note: This post was originally published in 2014, and has been updated with additional content in 2019.
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