Acceptable Quality Level or AQL is a statistical tool that is used to inspect a sample size for a give product in a lot and set the maximum number of acceptable defects. Alternatively, it is described as the worst tolerable process average when a series of lots samples are submitted for quality acceptance checks.
More recently, the term AQL has been renamed from "acceptable quality level" to "acceptable quality limit". This change was to increase the clarity of the description, which is the limit that is used to determine acceptable vs non-acceptable. In an ideal world, zero would be the preferred number of defects in products or services, however this is not the case. The acceptable quality limit is set based on business, financial, and safety levels.
AQL for products and services vary between different industries. For instance, companies that are providing medical tools require an extremely low tolerance and limit for AQL, because acceptance of defects could result in health and safety risks. Many companies generally choose between the cost of testing more strictly for AQL during production or risk a lower AQL tolerance with a potential for larger costs from product recalls. AQL is important for any company that is looking for Six Sigma level of quality control within their process.
The failure to meet the requirements of customers with respect to quality is termed as defects. The three categories of defects are:
Minor Defects - Not likely to reduce the usability and material of the product for its intended purpose, but slightly altered from the intended design and specifications. The industry average AQL for minor defects is 4%.
Major Defects - Not acceptable to the customers. Usage of the product will most likely result in failure. The industry average for AQL for major defects is 2.5%.
Critical Defects - If accepted, this type of defect could potentially harm or injury the customer. These defects are completely unacceptable. The industry standard for AQL for critical defects is 0%.
Please note that the number of defects is not the sole criteria for acceptance and refusal of a product. There are other criteria, such as design and regulations that would need to be accepted depending on the industry and customer needs.