Background on AQL

How the "Acceptable Quality Limit" works
The acceptable quality level (AQL) is a measure applied to products and defined in 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.
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 . For example, medical products are more likely to have more stringent AQL because defective products can result in health risks. 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).
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 of a .
AQL Defects
Instances of failure to meet customer quality requirements are termed as defects. In practice, there are three categories of defects:
Critical defects: Defects, 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%. (1)

Double Sampling
Double sampling plans were invented to give a questionable lot another chance. For example, if in double sampling the results of the first sample are not conclusive with regard to accepting or rejecting, a second sample is taken.
The double statistical sampling was created to give a greater level of depth to the results obtained from a simple sampling. This method is usually used for large statistical populations, and its use represents the study of additional variables to those obtained in simple sampling. This method is also usually called two-phase sampling. Its main benefit is to obtain more specific results and with less probability of errors.
Usually, double sampling is used when the results obtained on the basis of simple sampling are not presented as decisive, or when they leave doubts to the statisticians.
In this case, an additional sample is obtained from the same statistical population from which the first one was obtained, and the results are compared between them to analyze them and reduce the margin of error.
Double sampling is widely used in the evaluation of the characteristics of certain mass-produced material goods (such as toys) and in the quality control of companies dedicated to products susceptible to manufacturing errors.

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