BS ISO 3951-1-2013 pdf download.Sampling procedures for inspection by variables Part 1: Specification for single sampling plans indexed by acceptance quality limit (AQL) for lot-by-lot inspection for a single quality characteristic and a single AQL. 5.2 Use The AQL, together with the sample size code letter, is used to index the sampling plans in this part of ISO 3951. 5.3 Specifying AQLs The AQL to be used will be designated in the product specification or in the contract, or by the responsible authority. Where both upper and lower specification limits are given, this part of ISO 3951 addresses only the case of an overall AQL applying to the combined percent nonconforming beyond the two limits; this is known as combined control.N (See ISO 3951-2 for “separate” and “complex” control of double specification limits.) 5.4 Preferred AQLs The 16 AQLs given in this part of ISO 3951, ranging in value from 0,01 % to 10 % nonconforming, are described as preferred AQLs. They are only preferred in the sense that they are the AQL values used in the tabulations and charts. It follows that, if for any product or service, an AQL other than a preferred AQL is designated, then this part of ISO 3951 is not applicable. (See 142.) 5.5 Caution From the definition of the AQL in LI, it follows that the desired protection can only be ensured when a continuing series of lots is provided for inspection. 5.6 Limitation The designation of an AQL shall not imply that the supplier has the right to supply knowingly any nonconforming product. 6 Switching rules for normal, tightened, and reduced inspection Switching rules discourage the producer from operating at a quality level that is worse than the AQL. This part of ISO 3951 prescribes a switch to tightened inspection when inspection results indicate that the AQL is being exceeded. It further prescribes a discontinuation of sampling inspection altogether if tightened inspection fails to stimulate the producer into rapidly improving his production process. Tightened inspection and the discontinuation rule are integral and, therefore, obligatory procedures of this part of ISO 3951 if the protection implied by the AQL is to be maintained. This part of ISO 3951 also provides the possibility of switching to reduced inspection when inspection results indicate that the quality level is stable and reliable at a level better than the AQL. This practice is, however, optional (at the...
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