What is AQL and What Do You Need to Know About It?

by | Aug 30, 2018 | AQL, Product Inspections, Quality Control

If you’re preparing to conduct product inspections, you may have come across the term AQL.

Perhaps you’ve heard it mentioned in discussions with your inspection company or in articles you’ve read online.

What is AQL?

AQL stands for “Acceptable Quality Limit,” and it’s an important standard that’s used in the quality control industry. It’s defined in ISO 2859-1 as:

“The quality level that is the worst tolerable,” over the course of many inspections.

Why are Acceptable Quality Limits Important?

You and your inspection company can use the AQL standard to arrive at two important numbers:

  1. The number of units to inspect in a given shipment, and
  2. The number of defective units that will trigger a “fail” result.

The number of units that you inspect is referred to as the “inspection sample.”

To arrive at these numbers, you can use the two AQL tables that make up the AQL chart. Let’s take a look at the chart and how it works.

The AQL Chart: How it Works

The AQL chart is used to determine how many units should be inspected and how many defects are acceptable during your inspection.

The chart is broken down into two tables. You can see it in full resolution here.

Let’s say that you’ve placed an order with your factory for 1,500 plastic cups. We’ll start by looking at the first table, “Sample Size Code Letters.” You can see it below.

Table 1 – Sample Size Code Letters

AQL Table #1 - Example

In our hypothetical example, your order quantity is 1,500. So, it falls between 1,201 and 3,200 (underlined in the “Lot or Batch Size” column). Your “General Inspection Level” is level II (also underlined). As a rule of thumb, this is the inspection level that you should use most of the time.

We’ll talk more about the other levels later.

For now, we can line up your selection from the lot or batch size column with your general inspection level. When we do, we can see that your sample size code letter is “K,” which is circled.

We’ll use this code letter in the next table.

Table 2 – Single Sampling Plans for Normal Inspection

Looking at Table 2, we first focus on the “Sample Size Code Letter” column.

AQL Table #2 - Example

In this column, we’ll choose your sample size code letter, which we determined was “K.” Shifting our eyes to the second column, we can see that your sample size for the inspection is 125 units.

It is important to note that these units should always be chosen randomly. Random sampling is required to ensure the most accurate inspection results.

The columns further to the right are the different AQL levels. With most 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.

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. 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 developing quality standards. It covers these three types.

Now that you understand how to use the chart, let’s talk a little bit more about the general inspection levels.

General Inspection Levels: What You Need to Know

As we’ve already discussed, Table 1 lists three general inspection levels. They are General Levels I, II, and III. These levels are the main determinants of how many units will be in your random sample.

General Inspection Levels - I, II, III

In our example, you chose Level II (the standard level). As a rule of thumb, this is what you should choose most of the time.

If you had chosen Level I, the inspector would look at fewer units, only inspecting 50 plastic cups instead of 125. Sometimes, inspecting fewer units can lead to a reduction in cost. But you should really only use this level 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 for Level III. In this case, the inspector would look at 200 cups. An inspection with a larger sample size will take more time and effort to complete, but you can be more certain about the results.

Now we’ve talked about the General Inspection Levels, but you may be wondering about the Special Inspection Levels on the chart

Special Inspection Levels: What They are and When to Use Them

To the left of the General Inspection Levels are the Special Inspection Levels. There are four of them, S-1, S-2, S-3, and S-4.

Special Inspection Levels

The important thing to note about the special inspection levels is that selecting one will lead to a smaller sample size. For example, using Special Level S-1 on a 1,500 unit order would lead to the inspector only checking 5 units instead of the 125 they’d inspect at General Level II.

Why might you use the Special Inspection Levels?

The main reason is that, sometimes, it’s not feasible to inspect more products. For example, if:

  1. Testing is destructive to your product,
  2. Testing is very technical or time-consuming, or
  3. There are very few defects that actually concern you, and you need to move quickly.

In these cases, a Special Inspection Level might be appropriate for you. For more information about the Special Inspection Levels, take a look at this post.

Here we’re talking about inspecting very few products. But what about the opposite extreme?

Why not just inspect them all?

Why is Using AQL Better Than Conducting a 100% Inspection?

This is a common question that customers have for us. You may think, “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 important to keep in mind that, as your order quantities go up, it becomes less feasible. 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 wide 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. That’s not its purpose.

Can AQL Help Guarantee That My Shipment Has Zero Defects?

AQL stands for “acceptable quality levels.” In essence, it is not designed to ensure zero defects.

It would actually be unreasonable to expect, for most types of products, that your shipments will never have defects. With AQL, you are choosing the percentage of defects that you are willing to accept.

To reiterate, the standard defect levels are 2.5 major defects and 4.0 minor defects. To achieve lower defect levels, you can choose lower AQLs on Table 2 of the AQL chart.

Keep in mind, though, that this will require more units to be inspected, leading to higher inspection costs. It will also take more time to complete inspections.

Not to mention that your factory has to understand and agree to your expectations in the first place.

The AQL Standard Helps Ensure Good Quality Inspections

AQL or Acceptable Quality Limits are an important tool to help you and your inspection services provider determine how to conduct inspections. You can use the standard AQL chart to determine the sample size to inspect and the number of rejected units that are acceptable.

The General Inspection Levels can be used in most cases. But, if inspecting a large number of units isn’t feasible, the Special Inspection Levels can be used.

It’s important to keep in mind that using AQLs is a great alternative to conducting 100% inspections. Such inspections would be costly and time-consuming. They are also generally unnecessary unless your order size is small.

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 important. AQL determines the sample size for your inspections and understanding it is important. Having a good grasp of this method allows you to work with your third-party inspectors more effectively. This white paper will teach you all the basics of the AQL method.

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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