1 Scope
This International Standard specifies sequential sampling plans and procedures for inspection by variables of discrete items.
The plans are indexed in terms of producer’s risk point and the consumer’s risk point. Therefore, they are suitable not only for the purposes of acceptance sampling, but for the more general purpose of the testing of simple statistical hypotheses for proportions.
The purpose of this International Standard is to provide procedures for the sequential assessment of inspection results that may be used to induce the supplier 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 a lot (or process) of poor quality.
This International Standard is primarily designed for use under the following conditions:
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a) where the inspection procedure is to be applied to a continuing series of lots of discrete products all supplied by one producer using one production process. In such a case, sampling of particular lots is equivalent to the sampling of the process. If there are different producers or production processes, this International Standard shall be applied to each one separately;
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b) where only a single quality characteristic x of these products is taken into consideration, which must be measurable on a continuous scale;
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c) where the measurement error is negligible (i.e. with a standard deviation no more than 10 % of the process standard deviation);
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d) 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;
CAUTION — The procedures in this International Standard are not suitable for application to lots that have been screened previously for nonconforming items.
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e) where a contract or standard defines an upper specification limitU, a lower specification limitL, 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:
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1) x ≤ U (i.e. the upper specification limit is not violated);
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2) x ≥ L (i.e. the lower specification limit is not violated);
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3) x ≤ U and x ≥ L (i.e. neither the upper nor the lower specification limit is violated).
Inequalities 1) and 2) are called cases with a “single specification limit”, and 3) is the case with “double specification limits”.
In this International Standard, it is assumed that, where double specification limits apply, conformance to both specification limits is either equally important to the integrity of the product or is considered separately for both specification limits. In the first case, it is appropriate to control the combined percentage of product outside the two specification limits. This is referred to as combined control. In the second case, nonconformity beyond each of the limits is controlled separately, and this is referred to as separate control.