What is an inspection level in ISO 2859-1 and ANSI_ASQ Z1.4_

This article introduces the different options available to buyers when it comes to the representation of product inspection findings. Inspection level II (under “normal severity”) is appropriate for most random quality inspections. But it is sometimes necessary to increase–or reduce–the number of samples to check.

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.

The question becomes: How many products to check?


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.

Quality Control Tips

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 used standard for quality control in the form of acceptance sampling). By the way, that standard has formally been replaced by various commercial standards (ISO 2859-1, ANSI/ASQ Z1.4, and so forth).

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 an 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 fewer 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 opt for level-III inspections for high-value products. It can also be interesting for small quantities, where the inspection would take only one day whatever level is chosen.


Watch this video and learn how to improve management of quality from Chinese & Asian suppliers


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” (ISO 2859-1 standard).

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 container-loading supervision–to have an idea of what is inside the cartons, without spending too much time at that checking.


Two examples to get a clearer understanding

Let’s say you have ordered 5,000 pcs of a product. In the table below, you can see how many samples would be drawn under each of the 7 inspection levels.

General inspection levelsSpecial inspection levels

As you can see, the numbers of samples to check vary from 5pcs 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 you have ordered 40,000pcs of a product. Again, you can see the differences in sample sizes.

General inspection levelsSpecial inspection levels

In this case, the inspection might take one day of work (for S-1, S-2, S-3, S-4, or reduced level), two days (under level II), or three days (under level III).


What questions or issues do you have with the inspection levels? Are you currently suffering from quality from Chinese or Asian suppliers that isn’t reaching your expectations? Leave us a comment and we’ll be pleased to respond!

Editor’s note: This blog post was originally published on qualityinspection.org here but has been republished and edited for Sofeast readers here.


Get even more information on this topic from these posts:

Why we shouldn’t check 10% of the order quantity

The different types of sampling plans for QC inspections

What is the “AQL” (Acceptance Quality Limit) in simple terms?

About Renaud Anjoran

Our founder and CEO, Renaud Anjoran, is a recognised expert in quality, reliability, and supply chain issues. He is also an ASQ-Certified ‘Quality Engineer’, ‘Reliability Engineer’, and ‘Quality Manager’, and a certified ISO 9001, 13485, and 14001 Lead Auditor.

His key experiences are in electronics, textiles, plastic injection, die casting, eyewear, furniture, oil & gas, and paint.

Get help from Sofeast with quality inspections in China & Asia
This entry was posted in Quality Inspections and tagged , , , , , , , , . Bookmark the permalink.

10 responses to “What is an inspection level in ISO 2859-1 and ANSI/ASQ Z1.4?”

  1. Helena says:

    Hi Guys,

    We are doing a destructive test on bottles by measuring the torque to open value. Our lot size is 400. Is it AQL special levels that are used? What sample size would you recommend?

  2. Ruby says:

    Hi guys,

    We are doing a sampling test for nitrate inspection for papaya using merck RQflex. Procedures for determining papaya’s nitrate: peeling->cutting into halves->cut into smaller pieces->blend->strain->collect its volume. Our lot size depends on how many batch delivered (usually around 150,000).
    We already used general inspection level 1 before but we wanted to have a smaller sample size. I am thinking of using special inspection level but I am having a hard time on what special inspection level should I use and WHY. Can you help me with this one?

    • Renaud Anjoran says:

      You should simply pick the special level that you can realistically follow. You are the responsible authority that can make that call. The ISO 1859-1 standard doesn’t give us any rules to follow here. (You know that your ability to detect issues will be lower, as you go lower into the levels.)

  3. Pete says:

    We are testing nitrate for papayas and we already used general inspection level 1 (reduced inspection), but the manufacturer wants to have smaller sample size. I was thinking of using special inspection level but I don’t know what special inspection level should I use and WHY. I can’t seem to find the exact uses for each special inspection levels. I hope you can help with this

    • Renaud Anjoran says:

      Don’t look for a justification for using such or such special inspection level… Pick the one that you can realistically follow.

  4. Haru says:

    Hello, I’m Haru.

    We are searching for the appropriate special level of destructive test, of which lot size is 11,000.
    In this case, is it acceptable that S-1 or S-2 can be applied?

    In addition, there is any example of special level applied? I couldn’t find the detail criteria to determine the level of statistic quality control?

  5. Amber says:

    Hi! I am Amber
    I am currently looking for a strong reference such as ISO, GMP, etc that stated a sentence that has the same meaning as “if the sample is inspected through a destructive manner, a special level inspection can be carried out”, I would like to change the inspection level from general level 2 to special level (especially levels 2 and 3) for the test related to samples for product brochures through a destructive way. I’ve looked for it in ISO 2859-1 and ANSI/ASQ Z1.4, but the details regarding in what condition can I choose the special inspection level and its grade ( among S1/S2/S3/S4) are still missing. Do you have any information about the reference that i mean?

    • Sofeast says:

      Hi Amber, the standards don’t give any precise guidance… it’s all up to the authority (typically the quality manager).

Leave a Reply

Your email address will not be published. Required fields are marked *