ISO 3951-2:2006 specifies an acceptance sampling system of single sampling plans for inspection by variables, indexed in terms of the Acceptance Quality Limit (AQL). ISO 3951-2:2006 is primarily designed for use 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 the quality characteristics of the items of product are measurable on a continuous scale; where the measurement error is negligible (i.e. with a standard deviation no more than 10 % of the corresponding process standard deviation); where production is stable (under statistical control) and the quality characteristics are distributed, at least to a close approximation, according to normal distributions; where, in the case of multiple quality characteristics, the characteristics are independent of one another, at least approximately; where a contract or standard defines an upper specification limit, a lower specification limit, or both on each of the quality characteristics.