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
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).
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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.