This International Standard specifies sequential sampling plans and procedures for inspection by variables of discrete items. It is complementary to ISO 8422. The plans in the main body of the standard are indexed in terms of the producer’s risk point and the consumer’s risk point.
Annex A specifies sequential sampling plans and procedures indexed in terms of the acceptable quality level (AQL) to supplement the system of sampling plans in ISO 3951.
The purpose of this International Standard is to provide procedures based on a sequential assessment of inspection results, that may be used to induce the supplier through the economic and psychological pressure of non-acceptance of lots of inferior quality to supply lots of a quality having a high probability of acceptance. At the same time the consumer is protected by a prescribed upper limit to the probability of accepting lots of poor quality.
The sampling plans in this International Standard are primarily designed for use when all of the following conditions are satisfied:
- where the inspection procedure is to be applied to a continuing series of lots of discrete items all supplied by one producer using one production process. If there are different producers, this International Standard shall be applied to each one separately;
- where only a single quality characteristic x of these items is taken into consideration, which must be measurable on a continuous scale. If several such characteristics are of importance, this International Standard does not apply;
- where production is stable (under statistical control) and the quality characteristic x has a known standard deviation and is distributed according to a normal distribution or a close approximation to the normal distribution;
- where a contract or standard defines an upper specification limit U, a lower specification limit L, or both; an item is qualified as nonconforming when its measured quality characteristic x satisfies one of the following inequalities:
x > U ... (1.1) x < L ... (1.2)
either
x > U
or
x < L ... (1.3)