This document specifies an acceptance sampling system of single sampling plans for inspection by variables, primarily designed for use under the following conditions:
a) where the inspection procedure is 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 this process is taken into consideration, which is measurable on a continuous scale;
c) where the quality characteristic, x, is distributed according to a normal distribution or a close approximation to a normal distribution;
d) where the quality characteristic can be measured without error or with moderate measurement error;
e) 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 cases with a single specification limit, whereas inequality 3) is a case with double specification limits.
Where double specification limits apply, it is assumed in this document 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 fraction of a product outside the two specification limits. This is referred to as combined control.