The Ultimate Guide to AQL - Acceptable Quality Level
Ensure that your products meet pre-defined quality standards
What is an Acceptable Quality Level (AQL)?
The Acceptable Quality Level (AQL) is a key metric in quality control, crucial for determining the maximum number of defective items considered acceptable in a batch. Defined by ISO 2859-1, the AQL sets a benchmark for evaluating product quality during random sampling inspections. This limit helps manufacturers and buyers agree on defect levels that are tolerable for various products. AQL is expressed as a percentage or ratio, indicating the proportion of defects in relation to the total production quantity.
What is ISO 2859-1?
ISO 2859-1 is widely used in manufacturing and other industries to ensure product quality while minimizing inspection costs. It helps organizations balance the risks of accepting defective items against the costs of extensive inspections.
ISO 2859-1 is an international standard that specifies a set of procedures for sampling by attributes. It is commonly used in quality control to determine whether to accept or reject a batch of products based on the inspection of a sample. The standard is part of the ISO 2859 series, which covers different aspects and methods of acceptance sampling.
There is an 87-page document that discusses the sampling procedures for inspections by attributes. The document contains the follow sections:
- Sampling Plans: It provides various sampling plans that define the sample size and acceptance criteria based on the lot size and the acceptable quality level (AQL).
- Inspection Levels: The standard includes different inspection levels (e.g., normal, tightened, reduced) that adjust the stringency of the sampling plan based on historical quality performance.
- Switching Rules: These rules dictate when to switch between different inspection levels, depending on the outcomes of previous inspections.
- Tables and Charts: The standard includes tables and charts to help determine the appropriate sampling plan for a given situation.
AQL Categorization
AQL defects are categorized based on their severity in failing to meet customer quality standards:
How to read an AQL table
Acceptable Quality Level (AQL) is a global standard used to assess product quality through sample inspection from each batch. It defines the maximum number of defective items allowed in a sample before the entire batch is deemed unacceptable.
Products in a sample are tested randomly. If the defects are within the acceptable limit, the batch meets the AQL criteria. If not, manufacturers must analyze production processes to identify and address defect sources.
For instance, an AQL of 1% on a production run means that no more than 1% of the items can be defective. In a batch of 1,000 items, only up to 10 defective units are acceptable. If 11 or more are found defective, the batch fails to meet quality standards, hitting the rejectable quality level (RQL).
AQL is crucial for companies aiming for a Six Sigma level of quality control, a strategy introduced by Motorola in 1986. This approach strives for minimal defects and reduced variability to achieve exceptionally high-quality standards.
Six Sigma Level of Quality Control
Six Sigma is a quality control methodology aimed at reducing defects and improving processes. It uses statistical tools and techniques to drive process improvement.
By achieving high sigma levels, organizations can ensure that their processes are highly efficient and produce minimal defects, leading to better quality outcomes.
AQL - Example of how it works
AQL tables, also known as AQL charts, are used to determine the acceptable number of defects in a manufacturing process according to a specific standard. These tables are part of ISO 2859 and help in assessing how many defects are permissible under a given AQL level.
For instance, if a company orders 30,000 hats from a clothing manufacturer to be produced in one batch, and they agree on an AQL of 0.0 for critical defects, 3.0 for major defects, and 5.0 for minor defects, the AQL tables will guide them in determining the number of hats to inspect to ensure compliance with these standards during production.
To determine if AQL is met, you need the batch size, inspection type, inspection level, and desired AQL. Online calculators can assist with this process. For example, with a batch size of 50,000, a general inspection type, inspection level 2, and an AQL of 2.5, you would inspect a sample size of 500 units and allow no more than 21 defects. If more than 21 defects are found, the batch is rejected.
AQL Defects
- Critical Defects: These are unacceptable as they could harm users. The AQL for critical defects is usually set at 0%.
- Major Defects: These defects are generally unacceptable to end-users as they may lead to failure. The AQL for major defects is typically set at 2.5%.
- Minor Defects: These do not significantly impact product usability but deviate from specified standards. Some end-users might accept these products, with an AQL commonly set at 4%.
An AQL of 2.5 means that up to 2.5% of an order can be defective while still being acceptable. If the defect rate exceeds 2.5%, the batch fails to meet the quality standard agreed upon by the buyer and producer. For example, in an order of 20,000 pairs of shorts, up to 500 defective items would be considered acceptable under an AQL of 2.5%. An AQL of 2.5 means that up to 2.5% of an order can be defective while still being acceptable. If the defect rate exceeds 2.5%, the batch fails to meet the quality standard agreed upon by the buyer and producer. For example, in an order of 20,000 pairs of shorts, up to 500 defective items would be considered acceptable under an AQL of 2.5%.
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