What is the Acceptable Quality Level (AQL)?
The Acceptable Quality Level (AQL) is a quality control concept. It is the minimum level of faults acceptable in a sample of a manufactured product for the entire batch of the product to be accepted. If the number of faults is higher than the AQL, then the entire batch is rejected.
The acceptable quality level 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.
An acceptable quality level is a test and/or inspection standard that prescribes the range of the number of defective components that is considered acceptable when random sampling those components during an inspection. The defects found during an electronic or electrical test, or during a physical (mechanical) inspection, are sometimes classified into three levels: critical, major and minor:
- Critical defects are those that render the product unsafe or hazardous for the end user or that contravene mandatory regulations.
- Major defects can result in the product’s failure, reducing its marketability, usability or saleability.
- Minor defects do not affect the product’s marketability or usability, but represent workmanship defects that make the product fall short of defined quality standards.
Different companies maintain different interpretations of each defect type. In order to avoid argument, buyers and sellers agree on an AQL standard, chosen according to the level of risk each party assumes, which they use as a reference during pre-shipment inspection.
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.
The process of sampling requires that some preliminary parameters be fixed, including the AQL that would feed into the final decision about accepting or rejecting a batch of product. The parameters are:
1. Inspection Level
The three levels of inspection levels are normal, tightened, and reduced. Usually, the normal level is used to assess samples for faults. The level depends on the performance of the supplier. Consistently poor performance leads to tightened inspection, which is a stricter process. A good track record, on the other hand, leads to a reduced inspection level.
2. Acceptable Quality Level
The acceptable quality level is the number of allowable defects in a batch. The AQL is agreed upon between the buyer and the supplier based on the importance of the product. For example, the AQL for a batch of t-shirts can be 1 per 1,000, whereas, for a batch of mobile phone batteries, it is much lower, like 0.001 per 1,000 or 1 per 1,000,000 faults.
3. Sampling Method
A few sampling methods can be used to achieve quality assurance based on the level of sophistication and rigor required. The key methods are as follows:
Single sampling is the most common method. It only requires one sample of size n and the number of defaults c. Hence, it is also called (n,c)-sampling. If in the sample of n defaults are greater than c, the entire batch is rejected.
Double sampling is an extension of the single sampling method. In such a case, if the first sample is indecisive, then a second sample is taken to make the decision.
Sequential sampling is an elaborate method where each item from the sample is tested, and a decision to accept, reject, or continue testing is made after every item is tested.
The other methods are multiple sampling and skip lot sampling.
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 and set acceptable quality limits based on factors typically related to business, financial, and safety concerns.