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ISO 28590:2017 — Introduction to the ISO 2859 Series of Acceptance Sampling by Attributes
A comprehensive overview of attribute sampling schemes and plans for quality control, from AQL-indexed lot-by-lot inspection to skip-lot procedures.
Introduction to ISO 28590
ISO 28590:2017, prepared by ISO/TC 69/SC 5, provides a general introduction to acceptance sampling by attributes and a summary of the sampling schemes in the ISO 2859 series. It replaced ISO 2859-10:2006 as part of a renumbering initiative to create a more consistent portfolio within the 28590-28599 range. The standard is the gateway document for understanding when and how to apply the various parts of the ISO 2859 system of attribute sampling standards, which are among the most widely used statistical quality control tools in global manufacturing.
The ISO 2859 series is one of the most widely used quality management tools in manufacturing. ISO 28590 serves as the roadmap for navigating this family of standards.
Overview of the ISO 2859 Series
The ISO 2859 family comprises five principal standards, each designed for specific inspection scenarios. ISO 2859-1 provides AQL-indexed sampling schemes for continuing series of lots, with switching rules between normal, tightened, and reduced inspection that adapt to demonstrated quality performance. ISO 2859-2 provides LQ-indexed plans for isolated lots where no historical quality data is available. ISO 2859-3 offers skip-lot procedures when quality is consistently superior, enabling further reductions in inspection effort. ISO 2859-4 provides procedures for assessing declared quality levels — useful for regulatory audits and compliance verification. Finally, ISO 2859-5 provides sequential sampling plans within the AQL framework for situations where inspection costs are high.
| Standard | Title | Primary Application |
|---|---|---|
| ISO 2859-1 | Sampling schemes indexed by AQL | Continuing series of lots, lot-by-lot |
| ISO 2859-2 | Sampling plans indexed by LQ | Isolated lots, single batches |
| ISO 2859-3 | Skip-lot sampling procedures | Superior quality, reduced inspection |
| ISO 2859-4 | Declared quality level assessment | Audits, regulatory verification |
| ISO 2859-5 | Sequential sampling plans (AQL) | Costly or destructive testing |
Core Concepts: AQL, LQ, and Sampling Risk
The Acceptance Quality Limit (AQL) is the worst quality level that is still considered acceptable as a process average. The Limiting Quality (LQ) is the quality level that the consumer wants to reject with high probability. The standard explains the producer’s risk (rejecting a conforming lot) and consumer’s risk (accepting a nonconforming lot), and how the switching rules in ISO 2859-1 provide enhanced consumer protection without requiring excessively large sample sizes. When quality deteriorates, the switch to tightened inspection automatically increases the consumer’s protection; when quality is consistently excellent, reduced inspection lowers costs.
AQL is NOT a target quality level — it is a boundary. Operating at or near the AQL means accepting a significant probability (typically 5-10 %) of lot rejection. The target quality should be substantially better than the AQL.
Engineering Design Insights
From a quality engineering perspective, the most important insight from ISO 28590 is the distinction between AQL-indexed schemes (designed for continuing series) and LQ-indexed plans (designed for isolated lots). Applying the wrong type can lead to either excessive inspection costs or inadequate consumer protection. The switching rules in ISO 2859-1 — moving from normal to tightened inspection when quality deteriorates, and to reduced inspection when quality is consistently excellent — provide an adaptive framework that balances sampling economy with risk management over time.
Complementary Systems
The standard notes that a complementary system of sampling plans for inspection by variables, also indexed by AQL, is provided by the ISO 3951 series. Variables inspection often requires smaller sample sizes than attributes inspection for equivalent discrimination, but requires assumptions about the underlying distribution of the quality characteristic. The choice between attributes and variables inspection should be based on the nature of the quality data available and the cost of measurement.
Frequently Asked Questions
Q: What is the difference between AQL and LQ?
A: AQL is the acceptable process average quality level used for continuing series of lots. LQ is the limiting quality used for isolated lots — it represents the quality level the consumer wants to reject.
A: AQL is the acceptable process average quality level used for continuing series of lots. LQ is the limiting quality used for isolated lots — it represents the quality level the consumer wants to reject.
Q: When should skip-lot sampling be used?
A: Skip-lot sampling (ISO 2859-3) is appropriate when the process quality is consistently and demonstrably superior to the AQL over a long period, allowing reduced inspection frequency.
A: Skip-lot sampling (ISO 2859-3) is appropriate when the process quality is consistently and demonstrably superior to the AQL over a long period, allowing reduced inspection frequency.
Q: Can ISO 2859-1 plans be used for isolated lots?
A: Not without caution. ISO 2859-1 plans rely on switching rules across a continuing series of lots to provide consumer protection. For isolated lots, ISO 2859-2 with LQ indexing is more appropriate.
A: Not without caution. ISO 2859-1 plans rely on switching rules across a continuing series of lots to provide consumer protection. For isolated lots, ISO 2859-2 with LQ indexing is more appropriate.
Q: What is the relationship between ISO 28590 and the ISO 2859 series?
A: ISO 28590 is the introductory guide that explains the concepts, structure, and appropriate use of the entire ISO 2859 series of attribute sampling standards.
A: ISO 28590 is the introductory guide that explains the concepts, structure, and appropriate use of the entire ISO 2859 series of attribute sampling standards.
