What is AQL Sampling, and How is it Used in Quality Control Inspections?

by | Jul 6, 2021 | AQL, Product Inspections, Quality Control

AQL stands for ‘Acceptance Quality Limit,’ and it’s an essential sampling method used in quality control. It’s defined in ISO 2859-1 as “The quality level that is the worst tolerable” over the course of many inspections.

When an inspector goes to your manufacturer to conduct a product quality inspection, they use AQL sampling to answer two crucial questions:

  1. How many units should I inspect in a given shipment?
  2. How many defective products are allowable before the inspection fails?

When you import consumer products, it is vital to understand AQL so that you can work collaboratively with your inspectors. If you don’t understand the methodology fully, you risk misinterpreting your inspection results, which can lead to quality disasters.

Here we provide an overview of the AQL chart and how it works. You’ll learn how to use the AQL tables to answer the two questions above.

The Acceptance Quality Limit (AQL) Chart

The AQL chart is made up of two tables, which you can see below. They are also often referred to as the ANSI/ASQ Z1.4 tables. And you can use them consecutively to arrive at your sample size and determine the number of allowable defects per lot.


To download a full-resolution version, visit our AQL chart page.

These tables are based on the work of Harold F. Dodge and other statisticians who worked to improve them over the years. During World War II, Dodge worked at the Pentagon and developed quality control standards for inspecting US military equipment.

Today, importers use AQL to inspect all kinds of consumer products — from appliances to furniture to garments and more. In the next section, we’ll use reusable plastic cups as an example to help you understand how to use the tables.

How to Use the AQL Tables (Example: Inspecting Plastic Cups)

Reusable plastic cups

Let’s say that you’ve placed an order with your factory for 1,500 reusable plastic cups. You’ve hired a third party product inspection company (like Insight) to go to your factory before the order ships. You’ll use Table 1 to determine how many random samples they should pull.

Table 1 – Sample Size Code Letters

Since your purchase order includes 1,500 cups, you can look at the ‘Lot or batch size’ column on the left side of Table 1 and select the appropriate range. In this case, the correct lot size is from ‘1201 to 3200,’ underlined in red.

AQL Table #1 - Example

Then, you’ll select your inspection level, which determines how many products the inspector should check. You can see that we have underlined ‘General Inspection Level II’ in red. As a rule of thumb, you can use General II for most consumer products. We will discuss the other levels in more detail later.

For now, look for the intersection of your ‘Lot or batch size’ row with the ‘General II’ inspection level column. You’ll see that your ‘Sample size code letter’ is “K,” which we’ve circled in red. We’ll use this code letter in the following table.

Table 2 – Single Sampling Plans for Normal Inspection

Now that we know your ‘Sample size code letter’ from Table 1 is ‘K,’ we will select it on the left side of Table 2. One column over, under ‘Sample size,’ we can see that the inspector will need to check 125 randomly selected cups from your shipment of 1,500.

AQL Table #2 - Example

To the right of the ‘Sample size,’ you’ll see various columns representing different Acceptable Quality Levels. For most general consumer products, the standard AQL levels are 2.5% for major defects, 4.0% for minor defects, and 0% for critical defects. That’s why we’ve underlined ‘2.5’ and ‘4.0’ above.

Going by this standard, we can see that your inspector will accept a maximum of:

  • 7 major defects, and
  • 10 minor defects.

A shipment with 8 major defects, 11 minor defects, or 1 critical defect will fail inspection. So your inspector now has the guidelines they need to make a pass or fail decision. If you’re unsure of the difference between major, minor and critical defects, read our article about AQL defect classification.

3 Important Things to Understand About the AQL Chart

In our example above, we made certain assumptions. This first was that you would use General Inspection Level II in Table 1. The second was that you would use a 2.5% AQL level for major defects and a 4.0% level for minor defects. Below we will discuss some other alternatives.

#1 What are the General Inspection Levels? (Table 1)

General Inspection Levels - I, II, III

There are three General Inspection Levels — I, II, and III. The level you choose is the main factor that determines how many units to inspect. In our example, we decided on Level II, which is the most common selection for general consumer products. Because of this, your sample size came out to 125 cups.

If you had chosen Level I instead, the inspector would check fewer cups, only looking at 50 of them. On the other hand, if you had chosen Level III, they would inspect more, in this case, 200 cups.

Why might you choose Level I or III?

Sometimes, inspecting fewer units (Level I) can reduce costs since the inspection takes less time. The downside is that you can have less confidence in the result. You should only conduct a Level I inspection if you have a long history and degree of trust with your supplier.

On the other hand, if you are working with a new factory or have had a spate of recent failures, you might opt to inspect additional units (Level III). An inspection with a larger sample size will take more time and effort to complete, thus costing more, but you can have more confidence in the results.

For more information, take a look at our article about the General Inspection Levels.

#2 What are the Special Inspection Levels? (Table 1)

Special Inspection Levels

There are four Special Inspection Levels — S-1, S-2, S-3, and S-4. Using the Special levels will give you a smaller sample size than any of the General levels do. For example, if you use Special Level S-1 on a 1,500-unit order, the inspector will only check 5 cups instead of the 125 from General Level II.

Why Would You Use the Special Inspection Levels?

The Special Inspection Levels do not generally apply to the entire inspection. Instead, they only apply to specific types of tests on your quality inspection checklist. For example, you might use General II to assess workmanship — checking for scratches, smudges, or cracks in the product, and you might use Special S-1 to check the dimensions of the outer carton.

Typically, you can use the Special Inspection Levels when:

  • Testing is destructive to your product,
  • Testing is very technical or time-consuming, or
  • Certain defects are of low concern, and you need to move quickly.

For more information, take a look at our article about the Special Inspection Levels.

#3 What are the Various AQL Levels, Such as 1.0, 2.5, 4.0, etc.? (Table 2)

These different levels represent the AQL percentage you have chosen for your product. Keep in mind that they represent the maximum percentage of defects you are willing to tolerate over the course of many inspections.

The typical AQL levels for regular consumer products are:

  • 2.5% for major defects
  • 4.0% for minor defects
  • 0% for critical defects

‘Major’ defects are those that will make your product unsaleable or lead to customer returns, while ‘Minor’ defects are only slight deviations from your specifications. ‘Critical’ defects are those that might pose a safety hazard to the consumer. For more information about major, minor, and critical defects, see our article on AQL defect classification.

Some Frequently Asked Questions About AQL

Quality control can be a major concern for importers working with manufacturers in low-cost countries like China, India, or Vietnam. This often leads them to consider the following questions.

#1 Why Use AQL Rather Than Inspect 100% of My Products?

You may think to yourself, “A 100% inspection would give me 100% certainty that my products are ok.” If you are purchasing low volumes, this is reasonable. But it’s essential to keep in mind that it becomes less feasible as your order quantities go up. The amount of time it takes to complete an inspection goes up significantly, and the cost goes up as well.

AQL is a proven statistical model that has been used successfully for decades. It has broad adoption across the quality control industry for a reason — it works. However, it’s important to remember that conducting an AQL inspection will not guarantee a defect-free inspection, as this is not its purpose. For more information, see Should You Conduct a 100% Inspection or an AQL Sampling Inspection?

#2 Does AQL Guarantee That My Shipment Will Have Zero Defects?

AQL is based on the quality levels you deem to be most appropriate for your product. In essence, it is not designed to ensure zero defects. And it is generally unreasonable to expect zero defects outside of high-risk industries like aerospace or pharmaceuticals.

However, AQL is flexible and allows you to set higher standards for your brand. For example, you could choose an AQL level of 1.0% for major defects instead of the standard 2.5%, making your standards more stringent. This type of adjustment might be appropriate for a high-end brand. For more information about this type of adjustment, see How to Choose the Right AQL and Inspection Levels.

Additionally, we recommend downloading the following white paper if you would like to be sure you have a firm grasp on AQL inspections.

Free White Paper: AQL Inspections 101

As a consumer goods importer, the quality of your products is key to your success. Good quality products will earn you favorable reviews and repeat purchases. Thus, product inspections are essential. Having a good grasp of AQL sampling allows you to work with your third-party inspectors more effectively. Our AQL 101 white paper will teach you all the basics of the AQL method.
Please Note: This article was originally posted in August of 2018 and updated in July of 2021

Thanks for reading! Do you have any questions about the article? Is there something else you’d like to add? Leave a comment below.

Andy Church

Founder & CEO, Insight Quality Services

14 Comments

  1. Felix Tong

    For AQL Level II, 2.5, 4.0, quantity is 2,500, sampling size is 125. Then major accept 7 = 7/125 = 5.6%, minor accept 10 = 10/125=8%. This is not 2.5% for major and not 4% for minor.

    Please help to make me understand.

    Reply
    • Insight

      It is important to note the AQL does not equal the maximum percentage of defects that will be allowed in a sampling inspection. The AQL is the highest proportion defective that is considered acceptable as a long-run average for the process. Therefore a lot inspected to a 2.5% AQL may be passed by a Z1.4 sampling plan even if its true proportion defective is 5.0% or higher for majors; 4.0% AQL for minors can have a true proportion defect rate of of 8.0% or higher.

      Reply
  2. HopeThisHelps

    Maybe 3 comments to help users on this topic:

    – approximate definition of AQL value is “the percentage of defects in a batch where sample inspection has approximately 90-95% of chances to pass”. This means:
    . AQL is probabilistic. It i the key for proper understanding. 2 AQL on the same batch could provide 2 different results
    . AQL value measure a risk for the supplier, not a risk for the customer
    . even though a batch contains a quantity of defect equals to AQL value, there is still 5-10% to reject it
    . batches having a quantity of defect far above the AQL value still have a significant probability to be accepted. With a sampling plan K and AQL 2.5%, a batch containing 6% of defects still have 50% of chance to be accepted.

    – It should reminded to users that putting an AQL of 0.00% e.g. for critical defects is absolutely meaningless unless to perform 100% inspection of the batch. There is a common confusion between “critical defects” with the most stringent AQL and expected zero defects in a sample, and “redhibitory” defects where 0 defects is expected in full batch. Zero defect in a sample does not mean zero defect in a batch!

    – AQL is by design weak to perform quality control for release at a batch level due to its probabilistic nature, and should be more considered as strong tool to monitor consistency of a process overtime with consecutive AQLs on consecutives batches. More than a tool providing “magic answers” to release batches, AQL is somehow more designed to perform risk management on defects.

    HTH.

    Reply
    • Insight

      Thank you for sharing some thoughts. It is certainly true that zero defects found doesn’t mean a defect free batch. While AQL sampling inspections are not perfect, AQL sampling inspections are a proven methodology and a tool to provide buyers with a level of confidence that their order meets their quality expectations.

      Reply
  3. Jessica Rosik

    If your individual units come in shippers (say 10 units per shipper), how do you determine how many shippers to choose from?

    Reply
  4. theja

    Sir, Very useful and I have used your notes for explaining the AQL /Sampling scheme to ERP end users. Here my doubt is , in second table for ‘K’ = 125 units, if my AQL is 0.25% what is my Ac& Re, since it is showing Arrow marks instead of value. Which arrow mark ( upper or lower ) considerable?. As per my knowledge it may Ac1 & Re2 .Please correct me sir.

    Reply
    • Insight

      You can refer to the notes on the bottom of the chart. If there is a down arrow, use the first sampling plan below arrow; for an up arrow use the first sampling plan above the arrow. In your example of Sample Size K/125 pieces at .25 AQL the accept = 1; reject = 2
      https://insight-quality.com/resources/aql-chart/

      Reply
  5. ekka

    According to AQL, could you please tell, how to precise the number of measuring samples per size for a textile inspection?
    Is there any standard or its determined by customers?

    Reply
    • Insight Team

      The determination of what inspection level to use is up to you. Generally for measurements we suggest using a special level, S2 for example.

      Reply
  6. Liam

    Hi Insight, in response to your response to Theja’s comment above, should the sample size not move to 200 also?

    Reply
    • Insight Team

      Yes, the sample size should be adjusted to use the first sampling plan below the arrow. If the sample size is equal to or more than the batch size then 100% inspection should be conducted.

      Reply
  7. Fred Zilko

    Question regarding “acc” – “rej” in Table 2-A of the ISO 2859. We have 100 parts to be machined in 4 lots of 25 pcs. The requirement is AQL is 1.5. Our Insp. level is II, our code letter is “C” and our sample size is “5”.

    How do I interpret the “0” under the “acc” column. We’re only given one AQL and that is the 1.5.

    Does “0” mean no defects allowed”?

    OR

    Does is “acc” column interpreted to mean major” defect?

    Reply
    • Insight Team

      In your example of a lot/batch size of 25 and AQL 1.5 the sample size would be 8 because you would use the first sampling plan below the arrow. Accept 0/Reject 1 means 0 defects are allowed. The accept/reject numbers are not defined by major/minor defect classification and no assumption should be made as to whether it is major or minor defect.

      Reply

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