AQL, originally known as acceptable quality level, was redefined in 2008 as acceptable quality limit. ISO 2859 defines AQL as the “quality level that is the worst tolerable process average when a continuing series of lots is submitted for acceptance sampling.”
This article will describe the sampling and testing procedures for AQL as well the benefits and best practices of using an AQL approach.
Overview: What is AQL?
The original standard for assessing AQL was MIL-STD 105 E. This standard has formally been replaced by various commercial standards such as ISO 2859-1and ANSI/ASQ Z1.4.
ISO 2895 states the purpose of AQL is “to induce a supplier through the economic and psychological pressure of lot non-acceptance to maintain a process average at least as good as the specified acceptance quality limit, while at the same time providing an upper limit for the risk to the consumer of accepting the occasional poor lot.” It is measured in defect percent (or non-comformities) per hundred items.
AQL is a sampling methodology for doing incoming inspection of products. The decision to accept or reject lots of incoming materials is based on how many items can fail when sampled and inspected.
The AQL procedure provides three things during your inspection. They are:
- The number of units or sample size to inspect
- The number of units with minor defects resulting in a failed result
- The number of units with major defects resulting in a failed result
The type of defects can be categorized as:
- Critical defects: Accepted defects could harm customers. This kind of defect is not acceptable and is defined as 0% AQL.
- Major defects: Defects are not acceptable by the end users since they are likely to fail. The AQL for major defects is often 2.5%.
- Minor defects: Defects do not reduce the product’s functionality or purpose, and end users will still buy these products. The AQL for minor defects is usually 4%.
As long as sampled defective pieces remain within the AQL range, the incoming lot of materials will not be rejected. If the number of rejections is higher than the set AQL, the entire batch will be rejected. There are two tables that provide the necessary guidance to reject or not reject the batch.
The first table, shown below, helps you determine the appropriate sample size as a function of the lot size and desired inspection levels.
Note, there are three levels of General Inspection and four levels of Special Inspection. Based on previous experience with your vendor and the cost implications of sample size you would select the appropriate inspection level. The table provides the sample size code for determining whether to accept or reject the lot based on the number of defectives sampled from the lot.
Here are some guidelines for selecting your inspection level:
Level I (reduced inspection): Select this inspection level if you have had good experience with the vendor and they have been providing good quality in the past. This level requires less samples.
Level-II (normal inspection): Usually the default level.
Level-III (tightened inspection): If a supplier recently had quality problems, this level is appropriate since more samples will be inspected.
Special inspection levels (S-1, S-2, S-3, S-4) are normally used for certain types of on-site inspections or checks on a relatively small number of units in the lot. This is where an inspection requires destructive or time-consuming tests. The required sample sizes are smaller than with the general inspection levels.
The second AQL table, shown below, tells you how many defects or defectives you are allowed to find in your sample before you must reject the entire lot.
3 benefits of AQL
The use of AQL to inspect either outgoing or incoming materials is a proven method for deciding whether to accept or reject a lot of materials. It is relatively easy to implement and has some significant benefits.
1. Cost effectiveness
Random sampling is less expensive than 100% sampling.
You get to choose your general inspection level and acceptable AQL percentage.
3. Clear decision
Your decision to accept or reject is clear based on the values in Table 2 for major and minor defects.
Why is AQL important to understand?
While AQL is easy to use, there are some things you need to keep in mind.
Does not guarantee defect-free product
Since your sample size and decision criteria are based on a preselected level of quality, there is no assumption your lots will contain zero defects. The only expectation is that your quality will be no worse than your selected AQL defective rate.
Better than 100% inspection
In most cases, it’s not practical to inspect 100% of your product. Inspecting all your products is time-consuming and more expensive than sampling. The effectiveness of your inspectors decreases as they get tired.
The sample size and accept or reject values are statistically based. This gives you more confidence in your decision of what to do with the lot you have inspected.
An industry example of AQL
An electronics manufacturer was finding an excessive number of defects during an incoming inspection. They decided to start using ISO 2859 and AQL as a methodology to more effectively sample the components coming from their suppliers.
They tested their new approach with their largest supplier for a newly received lot size of 20,000 units. They chose a general inspection level II.
Since the 20,000 lot size fell between 10,0001-35.000 units on the left column, they needed to use a sample size in Table 2 associated with the letter M.
The sample size for the letter M from Table 1 is 315 units. They used an AQL of 2.5% for major defects and 4% for minor defects. Looking at the table above, you see the lot will be rejected if they find 15 or more major defects and 22 or more minor defects.
Unfortunately, inspection results found 13 major defects but 24 minor defects. Since that exceeded the allowable 21 units at the selected 4% AQL percentage, the lot was rejected.
3 best practices when thinking about AQL
Using AQL is a structured approach to sampling for acceptance. Here are a few hints to help optimize your use of the approach.
1. Pick an AQL and stick with it
Don’t send a signal that your AQL is negotiable and flexible.
2. Reward good suppliers
The default inspection level is II. If your supplier is doing a great job, consider using a less restrictive level.
3. Operational definitions and MSA
Be sure you have agreed-upon operational definitions of what a defect or defective is. Also be sure your measurement system is capable of identifying the defect or defective. Conduct a measurement system analysis (MSA) before implementing an AQL process.
Frequently Asked Questions (FAQ) about AQL
1. What if I find too many defects but want to accept the lot?
You can switch to 100% inspection and hope you find and remove all the defectives. You can have a manager decide whether the defects are significant enough to reject or whether they are actually acceptable.
2. Why are there different general inspection levels?
There are three general inspection levels referred to as I, II, or III. The default level is generally II. If your supplier is performing well, you can use the less restrictive Level I, which means smaller sample sizes. Level III is the most restrictive and requires a larger sample size.
The larger the sample size, the more confidence you will have in your decisions.
3. What is a common AQL percentage?
For critical defects, the AQL percentage is 0%. It is 2.5% for major defects and 4% for minor defects.
Summary of AQL
AQL is a popular sampling standard for choosing a sample size to be inspected, and the number of defects which will help determine whether to accept or pass an inspection lot of material. AQL’s random sampling approach is more time and cost-effective when compared to a 100% inspection of all of the units.
To determine the sample size to be inspected and the maximum allowed number of defect units, you use two AQL tables. The first table is used to find a letter code based on your lot size. The letter code is then used in the second table to find the sample size. The acceptable quality levels in the second table determine the maximum allowed number of defects.
The default general inspection level used in table 1 is II. An AQL level of 2.5 and 4.0 in table 2 are commonly used for major defects and minor defects, respectively. If the lot has either an excessive amount of major or minor defects, the lot is rejected.