Standards Development

Scope

This part of ISO 3951 is primarily designed for use under the following conditions:

a) where the inspection procedure is to be applied to an isolated lot of discrete products all supplied by one producer using one production process;

b) where only a single quality characteristic, x, of these process is taken into consideration, which must be measurable on a continuous scale;

c) where the quality characteristic, x, is distributed according to a normal distribution or a close approximation to the normal distribution; NOTE 1 The use of this part of ISO 3951 is applicable directly only when there is reason to believe that the distribution of measurements of the quality characteristic is normal (s. 11.1). If normal distribution is not warranted, data can be transformed (s. 11.2). In case of doubt, the responsible authority should be consulted.

d) where a contract or standard defines a lower specification limit, L, an upper specification limit, U, or both; an item is qualified as conforming if and only if its measured quality characteristic, x, satisfies the appropriate one of the following inequalities:

1) x L (i.e. the lower specification limit is not violated);

2) x U (i.e. the upper specification limit is not violated);

3) x L and x U (i.e. neither the lower nor the upper specification limit is violated). Inequalities 1) and 2) are called cases with a single specification limit and 3) a case with double specification limits.

Where double specification limits apply, it is assumed in this part of ISO 3951 that conformance to both specification limits is equally important to the integrity of the product. In such cases, it is appropriate to apply a single LQ to the combined percentage of a product outside the two specification limits. This is referred to as combined control

Purpose

This part of ISO 3951 specifies an acceptance sampling system of single sampling plans for isolated lot inspection by variables. It is indexed in terms of the limiting quality (LQ) for the inspection of lots where switching rules as used in ISO 3951-1 are not applicable. These switching rules provide protection to the consumer (by the prospect of switching to tightened inspection and discontinuation) and also provide an incentive to the supplier to improve the quality level. However, there are various cases where the switching rules of ISO 3951-1 are not applicable, such as isolated lots or a short series of lots. This part of ISO 3951 is complementary to ISO 2859-2 and designed for such cases.