- standard deviation
ISO 8423:2008 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 only a single quality characteristic 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 has a known standard deviation, and 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, a lower specification limit, or both. 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.
The procedures in ISO 8423:2008 are not suitable for application to lots that have been screened previously for nonconforming items.