Description of BS ISO 28593:2017 2017

1.1

This International Standard specifies a system of single sampling schemes for lot-by-lot inspection by attributes. All the sampling plans of the present system are of accept-zero form, i.e. no lot is accepted if the sample from it contains one or more nonconforming items. The schemes depend on a suitably-defined average outgoing quality limit (AOQL), the value of which is chosen by the user; no restrictions are placed on the choice of the value of the AOQL or on the sizes of successive lots in the series. The methodology ensures that the overall average quality reaching the customer or market-place will not exceed the AOQL in the long run.

1.2

The schemes are intended to induce a supplier, through the economic and psychological pressure of lot non-acceptance and consequent loss of accumulated credit, to attempt to maintain a nonconformity-free process, while assuring, by means of the lowest practicable sample sizes, that the long-term percentage of nonconforming items delivered to the customer or market-place does not exceed the AOQL. This objective is achieved by a progressive reduction in the sample size in response to good quality history.

1.3

The schemes are designed to be applied to a series of lots from each supplier. The credit principle provides:

1. automatic protection to the customer if a deterioration in quality is detected, by means of a total loss of accumulated credit and reversion to a relatively large sample size whenever a nonconforming item is found, and 100 % inspection of

   1. the first lot if it is not accepted, or

   2. any non-accepted lot that immediately follows a non-accepted lot;

2. an incentive to reduce sampling costs (by means of a progressive reduction in required sample size) if consistently good quality is achieved.

1.4

This International Standard is designed for use under the following conditions:

1. where the inspection procedure is to be applied to a series of lots of discrete items that are intended to be identical, and which are all supplied by one producer using one production process (If there are different producers or production processes, this International Standard is intended to be applied to each one separately.);

2. where one or more quality characteristics of these products are taken into consideration, which must all be classifiable as either conforming or nonconforming;

3. where the inspection error involved in classifying the state of a product's quality characteristic(s) is negligible;

4. where inspection is non-destructive.

This International Standard can be suitable for regulatory purposes, as control of the expected quality of items reaching the market-place is achieved with the smallest possible sample sizes, and long-term control of the realized, or actual quality level in the market-place is achieved with certainty, regardless of how long or short individual suppliers' series may be. This International Standard can be used by suppliers/producers, buyers/consumers and regulatory agencies to provide control of the expected quality of the totality of accepted product of each type from each source.

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