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Acceptable Quality Level (AQL): What It Is, How It Works

What is Acceptable Quality Level (AQL)?

The acceptable quality level (AQL) is a measure applied to products, defined in ISO 2859-1 as the “quality level that is the worst tolerable.” The AQL indicates the maximum number of defective components considered acceptable during random sampling quality inspections. It is typically expressed as a percentage or ratio of the number of defects relative to the total quantity.

Key points:

  • The acceptable quality level (AQL) represents the highest level of defects that is considered acceptable for a product.
  • The AQL varies depending on the product. Products with higher health risks will have a lower AQL.
  • Product batches that do not meet the AQL, typically assessed through a percentage measurement, are rejected during pre-shipment inspections.

How Acceptable Quality Level (AQL) Works

Products within a sample undergo random testing, and if the count of faulty items falls below a predetermined threshold, that product aligns with the acceptable quality level (AQL). However, when a specific sample fails to meet the AQL, manufacturers scrutinise various production parameters to identify the sources of defects.

For instance, envision an AQL set at one per cent for a production batch, where no defects must be found at over one per cent. Say in a batch of 1,000 products, only ten can be defective; if there’s more than ten, the entire batch is deemed unacceptable. This threshold of 11 or more defective products represents the rejectable quality level (RQL).

AQL Standards in Different Industries

The acceptable quality level (AQL) for a product can significantly differ across industries. For instance, industries like medical products often uphold more stringent AQL standards due to the potential health risks associated with defective items.

On the other hand, products with minimal consequences stemming from potential defects might maintain a less stringent AQL, such as a TV remote control. Companies need to balance the increased expenses linked to rigorous testing and the possibility of higher waste resulting from stricter defect allowances against the potential costs of a product recall.

Naturally, customers ideally prefer products or services devoid of defects, aiming for a flawless acceptable quality level. Nevertheless, sellers and consumers usually negotiate and establish acceptable quality thresholds considering various factors, including business, financial, and safety considerations.

What are AQL Tables?

AQL tables, also known as AQL charts, are created to provide a reference standard for defining an acceptable count of defects in manufacturing. They serve as a guide, indicating the permissible number of defects a company can have to meet a specific AQL. These tables are an integral part of ISO 2859.

For instance, suppose a clothing manufacturer receives an order for 30,000 hats to be produced in a single batch. The buyer and producer have settled on an AQL of 0.0 for critical defects, 3.0 for major defects, and 5.0 for minor defects. Consulting the AQL tables enables both parties to determine the required number of hats to inspect, ensuring compliance with the agreed-upon AQL throughout the production process.

AQL Defects

There are instances where customer quality standards aren’t met are categorised as defects. In practice, defects fall into three categories:

  1. Critical defects: These defects, if accepted, could pose risks to users and are considered unacceptable, with a defined AQL of zero per cent 
  2. Major defects: Typically, these defects are unacceptable to end-users as they’re likely to cause failures. The AQL for major defects is 2.5 per cent.
  3. Minor defects: These defects don’t significantly diminish the product’s usability for its intended purpose but deviate from specified standards. Some end-users may still purchase products with minor defects. The AQL for minor defects is set at four per cent.

AQL in Practice

Acceptable quality level (AQL)

Typically regarded as the highest quality level deemed satisfactory, AQL represents the maximum percentage of defects still considered acceptable. Ensuring a high probability of accepting an AQL lot is crucial. A probability of 0.95 equates to a 0.05 risk.

Rejectable quality level (RQL)

This signifies an unacceptable quality level, also known as Lot Tolerance Percent Defective (LTPD). Tables often standardise the consumer’s risk as 0.1 for RQL. The probability of accepting an RQL lot is notably low.

Indifference quality level (IQL)

Positioned between AQL and RQL, this quality level holds varying interpretations among different companies regarding each defect type. Nonetheless, buyers and sellers mutually agree on an AQL standard that aligns with the risk level each party assumes. These agreed-upon standards serve as a benchmark during pre-shipment inspections.

Factors for AQL Compliance Assessment

The compliance assessment for Acceptable Quality Level (AQL) involves some crucial factors that determine the adherence to quality standards:

  • Batch Size: The total quantity or volume of products in a batch plays a pivotal role in AQL compliance. Larger batch sizes often necessitate proportionally larger sample sizes for inspection to accurately represent the entire lot. Smaller batches might allow for more comprehensive inspections due to manageable sample sizes.
  • Inspection Type: The method or approach used for inspection significantly impacts AQL compliance. Different inspection techniques exist, such as attribute sampling or variable sampling, each suited to specific industries or product types. The choice of inspection type influences the effectiveness of detecting defects within the batch.
  • Inspection Level: This refers to the degree or intensity of the inspection process. It involves deciding how thoroughly the products within the batch will be scrutinised. Inspection levels range from general sampling to more extensive and detailed examinations, with higher inspection levels often leading to more stringent quality control.
  • Desired AQL: The predetermined acceptable quality level directly influences the inspection parameters. Determining the acceptable defect rate or percentage within a batch guides the inspection process and the number of defects allowable before rejecting the entire lot.

FAQs

What does an AQL of 2.5 mean?

An AQL set at 2.5 implies that only 2.5 per cent of the ordered items can have defects to meet the acceptable criteria. Should the defect rate exceed 2.5 per cent, it fails to meet the agreement between the buyer and producer. For instance, in an order of 20,000 pairs of shorts, no more than 1,250 defective pairs would align with an AQL of 2.5 per cent.

What is the standard AQL?

There exists no universal AQL standard; rather, AQL varies depending on the product and industry. For instance, the medical sector requires an exceptionally low AQL due to the potential harm defects in medical equipment or products can pose to consumers. In contrast, clothing manufacturing might tolerate a higher AQL. Ultimately, the agreed-upon AQL between the buyer and producer during the order placement determines the acceptable defect level.

Conclusion

The Acceptable Quality Level (AQL) is a valuable metric to ensure that large manufacturing orders meet the buyer’s required standards. It helps sellers efficiently manage their production processes while adhering to these standards. Typically used in large-scale production orders, AQL is instrumental in maintaining a balance between quality and efficiency, ensuring both buyer and seller satisfaction by producing high-quality products.

DISCLAIMER: This article is for informational purposes only.

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