What is the Acceptable Quality Level (AQL)?
The acceptable quality limit (AQL) is a measure applied to products and defined in ISO 2859-1 as the “most tolerable level of quality”. The AQL tells you how many defective components are considered acceptable during random sampling quality inspections. It is generally expressed as a percentage or ratio of the number of defects compared to the total quantity.
AQL for a product can vary from industry to industry; medical products, for example, have stringent AQLs because defective products pose a health risk.
Operation of the Acceptable Quality Level (AQL)
The goods in a sample are randomly tested and if the number of defective items is less than the predetermined quantity, this product is deemed to meet the acceptable quality level (AQL). If the acceptable quality level (AQL) is not reached for a particular sample of goods, the manufacturers will examine the various parameters of the production process in order to determine the areas causing the defects.
As an example, consider an AQL of 1% over a production cycle. This percentage means that no more than 1% of the lot may be defective. If a production cycle consists of 1000 products, only 10 products can be defective. If 11 products are defective, the entire lot is discarded. This figure of 11 or more defective products is known as the Rejectable Quality Limit (RQL).
The AQL is an important statistic for businesses looking for a Six Sigma level of quality control 503, which is a quality control methodology developed in 1986 by Motorola, Inc. The AQL is also known as the acceptable quality limit.
Key points to remember
- The Acceptable Quality Limit (AQL) is the worst level of tolerable quality for a product.
- The AQL differs from one product to another. Products that could cause more health risks will have a higher AQL.
- Batches of products that do not meet the AQS, generally based on a percentage measurement, are rejected when tested during preshipment inspections.
AQL for a product can vary from industry to industry. For example, medical products are more likely to have a more stringent AQL because defective products can pose health risks.
On the other hand, a product with mild side effects from a possible defect may have a less strict AQL, like the remote control of a television. Companies must weigh the additional cost associated with rigorous testing and potentially higher deterioration due to less acceptance of faults with the potential cost of a product recall.
Of course, customers would prefer flawless products or services, the ideal acceptable level of quality. However, sellers and customers generally try to arrive and set acceptable quality limits based on factors generally related to business, financial and security concerns.
Cases of non-compliance with customer quality requirements are qualified as defects. In practice, there are three categories of faults:
- Critical faults: Defects, when accepted, can harm users. Such defects are unacceptable. Critical faults are defined as 0% AQL.
- Major faults: Defects are generally not acceptable to end users, as they are likely to cause failure. The AQL for major faults is 2.5%.
- Minor faults: Defects which are not likely to significantly reduce the usability of the product for the intended purposes but which differ from the specified standards; some end users will still purchase such products. The AQL for minor faults is 4%.
Although it is called the “acceptable” quality level, the AQL is in fact the worst tolerable quality level on average over a period covering a certain number of batches.
AQL in practice
Acceptable quality level (AQL): AQL is generally considered to be the worst level of quality which is always considered satisfactory. It is the maximum percentage of faults that can be considered satisfactory. The probability of accepting an AQL batch must be high. A probability of 0.95 translates into a risk of 0.05.
Rejectable quality level (RQL): This is considered an unsatisfactory level of quality and is sometimes called defective batch tolerance (LTPD). The risk to the consumer has been normalized in some tables to 0.1. The probability of accepting an RQL batch is low.
Quality of indifference level (IQL): This level of quality is somewhere between AQL and RQL.
Different companies maintain different interpretations of each type of default. However, buyers and sellers agree on an AQL standard appropriate to the level of risk assumed by each party. These standards are used as a reference during a pre-shipment inspection.