Acceptable Quality Level (AQL)

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Acceptable Quality Level (AQL): A Concise Guide

What is AQL?

Acceptable Quality Level (AQL) is a quality-control statistic used in sampling inspections to define the maximum proportion of defective items in a production lot that can be considered acceptable. Defined in ISO 2859-1, AQL represents the “worst tolerable” quality level agreed between buyer and seller. It is usually expressed as a percentage or ratio (for example, 1% means up to 1% defective items are acceptable).

How AQL Works

  • Inspection is performed on a random sample drawn from a production lot.
  • Using an AQL sampling plan (ISO 2859-1 tables), you determine sample size and an acceptance number (the maximum defects allowed in the sample).
  • If the number of defects in the sample is at or below the acceptance number, the lot is accepted; if it exceeds the acceptance number, the lot is rejected.
  • AQL does not guarantee zero defects—it’s a trade-off between inspection cost and the risk of accepting defective items.

Example: For a 5,000-unit lot with an AQL of 1%, up to 50 defective units would be considered acceptable if inspection supported that result. Exact acceptance rules depend on the chosen sampling plan.

Defect Categories

Defects are typically categorized to reflect severity and different AQLs:

  • Critical defects: Could cause harm or violate regulations. Often set to AQL = 0% (no critical defects allowed).
  • Major defects: Likely to result in product failure or customer dissatisfaction. Common AQLs for major defects are often around 2.5–3.0%, though this varies.
  • Minor defects: Cosmetic or small deviations that do not significantly affect usability. Typical AQLs range from about 4–5%.

These categories let buyers and manufacturers set different tolerances for different kinds of problems.

Related Concepts

  • Rejectable Quality Level (RQL) / Lot Tolerance Percent Defective (LTPD): A level of defectiveness considered unacceptable; sampling plans are designed to have a low probability of accepting lots at the RQL.
  • Indifference Quality Level (IQL): A defect level between AQL and RQL where acceptance probability is neither strongly high nor low.
  • Operating Characteristic (OC) curve: Describes the probability of accepting lots at various true defect rates; used to understand consumer and producer risk.

Using AQL Tables (Sampling Plans)

  • AQL tables specify: lot-size ranges, sample sizes, and acceptance/rejection numbers for given AQLs and inspection levels.
  • You must decide: lot size, inspection type (normal, tightened, reduced), inspection level (I, II, III—II is commonly used), and the agreed AQL for each defect category.
  • Example: For a very large lot, inspection level II and an AQL of 2.5% might require a sample of roughly 500 units with an acceptance number around 21 (exact values come from the ISO 2859-1 tables).

Industry Variation and Choosing an AQL

  • AQL varies by product risk, regulation, and commercial negotiation.
  • High-risk products (medical devices, pharmaceuticals) use very low AQLs, sometimes close to zero.
  • Consumer goods with lower risk may use higher AQLs (e.g., 2.5–5% for some categories).
  • Factors influencing the choice:
  • Safety and regulatory requirements
  • Cost of inspection and potential spoilage
  • Cost and consequences of product failure or recall
  • Customer expectations

Practical Tips

  • Specify AQLs and defect categories in contracts and purchase orders to avoid disputes.
  • Use inspection level II for routine checks; tighten or reduce based on historic quality performance.
  • Remember that AQL is for lot acceptance; it is not a measure of process capability (use Six Sigma or process capability indices for continuous improvement goals).
  • When in doubt, for safety-critical items err on the side of stricter AQLs and more frequent inspections.

Key Takeaways

  • AQL defines the maximum proportion of defects acceptable in a lot during sampling inspection.
  • It balances inspection cost, production efficiency, and acceptable risk.
  • Use ISO 2859-1 sampling plans (AQL tables) to determine sample sizes and acceptance numbers.
  • Choose AQLs based on defect severity, product risk, and agreement between buyer and seller.

Sources

  • ISO 2859-1: Sampling procedures for inspection by attributes
  • Industry guidance on AQL application and inspection planning