What is AQL and What Do You Need to Know About It?
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:
- The number of units to inspect in a given shipment, and
- 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
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.
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.
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.
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:
- Testing is destructive to your product,
- Testing is very technical or time-consuming, or
- 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.