What Is Acceptable Quality Level (AQL)?
The acceptable quality level (AQL) is a measure applied to products and defined in ISO 2859-1 as the “quality level that is the worst tolerable.” The AQL tells you how many defective components are considered acceptable during random sampling quality inspections. It is usually expressed as a percentage or ratio of the number of defects compared to the total quantity.
- The acceptable quality level (AQL) is the worst quality level that is tolerable for a product.
- The AQL differs from product to product. Products that might cause more of a health risk will have a lower AQL.
- Batches of products that do not meet the AQL, typically based on a percentage measurement, are rejected when tested during pre-shipment inspections.
How Acceptable Quality Level (AQL) Works
Goods in a sample are tested at random, and if the number of defective items is below the predetermined amount, that product is said to meet the acceptable quality level (AQL). If the acceptable quality level (AQL) is not reached for a particular sampling of goods, manufacturers will review the various parameters in the production process to determine the areas causing the defects.
The AQL of a product can vary from industry to industry; medical products, for example, have stringent AQLs because defective products are a health risk.
As an example, consider an AQL of 1% on a production run. This percentage means that no more than 1% of the batch can be defective. If a production run is composed of 1,000 products, only 10 products can be defective. If 11 products are defective, the entire batch is scrapped. This figure of 11 or more defective products is known as the rejectable quality level (RQL).
The AQL is an important statistic for companies seeking a Six Sigma level of quality control, which is a quality-control methodology developed in 1986 by Motorola, Inc. AQL is also known as the acceptable quality limit.
Industry Differences in AQL Standards
The AQL of a product can vary from industry to industry. For example, medical products are more likely to have more stringent AQL because defective products can result in health risks.
In contrast, a product with benign side-effects from a possible defect may have a less strict AQL, such as the remote control for a TV. Companies have to weigh the added cost associated with the stringent testing and potentially higher spoilage due to a lower defect acceptance with the potential cost of a product recall.
Customers would, of course, prefer zero-defect products or services; the ideal acceptable quality level. However, sellers and customers usually try to arrive at and set acceptable quality limits based on factors typically related to business, financial, and safety concerns.
Using AQL Tables
AQL tables (also sometimes called AQL charts) are designed to allow users to reference a standard for what qualifies as an acceptable number of defects in manufacturing. They allow a person to see how many defects would be allowed for a company to achieve a certain AQL. The tables are part of ISO 2859.
So for example, let's say a company orders 30,000 hats from a clothing manufacturer that will be produced in one batch, and the buyer and producer have agreed on AQL 0.0 for critical defects, AQL 3.0 for major defects, and AQL 5.0 for minor defects. Referencing the AQL tables would allow the buyer and producer to determine how many hats would need to be inspected to ensure that the agreed upon AQL is being met during production.
Instances of failure to meet customer quality requirements are termed as defects. In practice, there are three categories of defects:
- Critical defects: Defects that, when accepted, could harm users. Such defects are unacceptable. Critical defects are defined as 0% AQL.
- Major defects: Defects usually not acceptable by the end-users, as they are likely to result in failure. The AQL for major defects is 2.5%.
- Minor defects: Defects not likely to reduce materially the usability of the product for its intended purpose but that differ from specified standards; some end users will still buy such products. The AQL for minor defects is 4%.
AQL in Practice
Acceptable quality level (AQL): AQL is typically considered to be the worst quality level that is still considered satisfactory. It is the maximum percent defective that can be considered satisfactory. The probability of accepting an AQL lot should be high. A probability of 0.95 translates to a risk of 0.05.
Rejectable quality level (RQL): This is considered an unsatisfactory quality level and is sometimes known as lot tolerance percent defective (LTPD). The consumer’s risk has been standardized in some tables as 0.1. The probability of accepting an RQL lot is low.
Indifference quality level (IQL): This quality level is somewhere between AQL and RQL. Different companies maintain different interpretations of each defect type. However, buyers and sellers agree on an AQL standard that is appropriate to the level of risk each party assumes. These standards are used as a reference during a pre-shipment inspection.
What Factors Are Used to Determine if AQL Is Being Met?
To calculate AQL, you need the lot or batch size, the inspection type, inspection level, and the desired AQL. There are AQL calculators available online. If your lot or batch size is 50,000, your inspection type is general, your inspection level is 2, and your AQL level is 2.5, you would need a sample size of 500 units with only 21 defects. Anything over that has reached the rejection point.
What Does an AQL of 2.5 Mean?
An AQL of 2.5 means that only 2.5% of an order can be defective in order for it to be acceptable. If more than 2.5% of the other is defective, then it doesn't meet the agreement between the buyer and producer. So for example, if 20,000 pairs of shorts are ordered, only 1,250 can be defective to reach an AQL of 2.5.
What Is the Standard AQL?
There is no standard AQL. AQL changes based on product and industry. For example, in the medical industry, AQL must be very low, because defects in medical equipment or products can be harmful to consumers. In clothing manufacturing, AQL might be higher. However, it all comes down to what the buyer and producer agree upon when making the order.
The Bottom Line
AQL can be a useful figure to help ensure that large orders for manufacturing are completed to a level of standard that a buyer demands. It also allows the seller to be efficient in their production while maintaining the standard set by the buyer. It is generally used in large-scale orders of production, and is helpful in keeping both buyer and seller happy while producing good quality products.