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.