Sampling procedures for inspection by variables -- Part 3: Double sampling schemes indexed by acceptance quality limit (AQL) for lot-by-lot inspection
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The objectives of the methods laid down in ISO 3951-3:2007 are to ensure that lots of acceptable quality have a high probability of acceptance and that the probability of non-accepting inferior lots is as high as practicable. This is achieved by means of the switching rules, which provide automatic protection to the consumer (by means of a switch to tightened inspection or discontinuation of sampling inspection) should a deterioration in quality be detected, and an incentive (at the discretion of the responsible authority) to reduce inspection costs (by means of a switch to a smaller sample size) should consistently good quality be achieved.
In ISO 3951-3:2007, the acceptability of a lot is implicitly or explicitly determined from an estimate of the percentage of nonconforming items in the process, based on either one or two random samples of items from the lot.
ISO 3951-3:2007 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 items of product have a single quality characteristic; where the quality characteristic is 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 characteristic is distributed, at least to a close approximation, according to a normal distribution; where the possibility of having to select and inspect a second sample is administratively acceptable; and where a contract or standard defines an upper specification limit, a lower specification limit, or both on the quality characteristic.
The procedures in ISO 3951-3:2007 are not suitable for application to lots that have been screened previously for nonconforming items.