The vast majority of inspectors follow the ISO 2859-1 standard. But about ISO 2859-2, -3, and so on?
The relationship between these parts is explained in ISO 2859-10:2006 (introduction to the ISO 2859 series of standards for sampling for inspection by attributes):
⎯ Part 1: Sampling schemes indexed by acceptance quality limit (AQL) for lot-by-lot inspection
⎯ Part 2: Sampling plans indexed by limiting quality (LQ) for isolated lot inspection
⎯ Part 3: Skip-lot sampling procedures
⎯ Part 4: Procedures for assessment of declared quality levels
⎯ Part 5: System of sequential sampling plans indexed by acceptance quality limit (AQL) for lot-by-lot inspection
⎯ Part 10: Introduction to the ISO 2859 series of attribute sampling standards
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As I explained in this video, part 1 of ISO 2859 (based on the “AQL tables”) is not to be used systematically. It should be reserved for business with regular suppliers. The standard explains it this way:
ISO 2859-1 was developed primarily for the inspection of a continuing series of lots all originating from the same production or servicing process.
For one-shot orders, or for new suppliers, part 2 is to be used. The reason is, the switching rules can’t be used since there is not a regular flow of batches to inspect:
ISO 2859-2 provides sampling plans applicable for use when individual or isolated lots are to be sampled. These sampling plans are in many instances identical to those in ISO 2859-1.
Is it a big problem if part 1 (based on the AQL tables) is used in circumstances where part 2 is actually applicable? Not really.
Sampling plans in ISO 2859-1 may also be used for the inspection of lots in isolation, but in this case, the user is strongly advised to consult the operating characteristic curves to find a plan that will yield the desired protection.
When to use part 3, and check only random batches rather than all batches? For very good suppliers only:
ISO 2859-3 provides skip-lot procedures for use when the process quality is markedly superior to the AQL for a defined long period of delivery or observation.
And what about part 4?
ISO 2859-4 provides a procedure that may be used to verify a quality level that has been declared for some entity.
The procedures in ISO 2859-4 have been developed in response to the need for sampling procedures suitable for formal, systematic inspections such as reviews or audits.
And part 5, which is particularly complex?
ISO 2859-5 provides a method of establishing sequential sampling plans of discriminatory power essentially equivalent to that of corresponding plans of ISO 2859-1.
In conclusion, QC practitioners tend to make their job easy and simplify the rules to follow. I think it is, to a large extent, not a problem. The main issue is that very few people follow the switching rules (which influences the number of samples and/or the limits for nonconformities, based on a score earned by each manufacturer).
By the way, are you wondering what the difference is between “inspection by attributes” and “inspection by variable”? See The different types of sampling plans for QC inspections.
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