scope:

ISO 3951-1:2005 specifies an acceptance sampling system of single sampling plans for inspection by variables, in which the acceptability of a lot is implicitly determined from an estimate of the percentage of nonconforming items in the process, based on a random sample of items from the lot.
ISO 3951-1:2005 is primarily designed for application under the following conditions:
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;
where only a single quality characteristic ___x____ of these products is taken into consideration, which must be measurable on a continuous scale;
where the measurement error is negligible, i.e. with a standard deviation no more than 10% of the process standard deviation;
where production is stable (under statistical control) and the quality characteristic ___x____ 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 conforming if and only if its measured quality characteristic___ x____ satisfies the appropriate one of the following inequalities:
___x____ greater than or equal to ___L____ (i.e. the lower specification limit is not violated);
___x____ greater than or equal to ___U____ (i.e. the upper specification limit is not violated);
___x____ greater than or equal to ___L____ and ___x____ less than or equal to ___U____ (i.e. neither the lower nor the upper specification limit is violated).
The first two inequalities are called cases with a single specification limit, and the third a case with double specification limits.
If double specification limits apply, it is assumed in ISO 3951-1:2005 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 AQL to the combined percentage of product outside the two specification limits. This is referred to as combined control.