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:
- How many units should I inspect in a given shipment?
- 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)
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.
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.
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)
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)
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
You may have one or more of 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, so it must be the better choice.” And if you are purchasing low volumes, this is reasonable. However, full inspections become less feasible as your order quantities go up. The time to complete the inspection and its cost will increase.
AQL is a proven statistical model that has been used 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 What Happens When I Land on an Arrow in Table 2?
If the arrow points up, use the sampling plan above the arrow; if it points down, use the plan below the arrow. For example, let’s say you’ve ordered 90 sets of wireless headphones and decided on General II and 2.5% for major defects.
Your code letter is E, which lands you on a down arrow, so you move down to the lower sampling plan. You will inspect 20 sets, accepting the lot if there is 1 defect and rejecting it if there are 2 defects.
#3 Why Don’t the “Accept” Numbers Match My Selected Percentage?
Let’s say, for example, you have placed an order for 3,200 table and chair sets. You have selected General II and 2.5% for major defects — your code letter is K.
In this scenario, your sample size is 125, and you would accept 7 defects but reject the lot if there are 8 defects. 7 defects represent 5.6% of the 125 units you are inspecting, so why is this?
You should remember that AQL is a statistical tool and is probabilistic in nature. The 2.5% represents the expected long-term average defect rate over the course of many lots, not the percentage of defects in the sample size.
#4 How Many Cartons do You Pick the Samples From?
Let’s say you’ve ordered 1,000 units of your product and determined that you will inspect a sample size of 80. However, your goods are shipped in cartons of 10 units each, and you wonder how many cartons to pull samples from.
To determine this, the inspector always takes the square root of the total number of cartons plus one. Since there are 100 cartons, they pick 11 and then pull the samples from those.
#5 Does AQL Guarantee That My Shipment Will Have Zero Defects?
AQL is based on the quality levels you deem most appropriate for your product. In essence, it is not designed to ensure zero defects. It is generally unreasonable to expect zero defects outside 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 want to be sure you have a firm grasp of AQL inspections.
Free White Paper: AQL Inspections 101
Note: This article was originally posted in August of 2018 and updated in November of 2022