DANSK DS/ISO 3951-5 PDF

1.1 This part of ISO 3951 specifies a system of sequential sampling plans (schemes) for lot-by-lot inspection by variables. The schemes are indexed in terms of a preferred series of acceptance quality limit (AQL) values, ranging from 0,01 to 10, which are defined in terms of percent nonconforming items.
1.2 The schemes of ISO 3951 are intended to induce a supplier through the economic and psychological pressure of lot non-acceptance to maintain a process average at least as good as the specified AQL value, while at the same time providing an upper limit for the risk to the consumer of accepting the occasional poor lot.
1.3 The schemes are designed to be applied to a continuing series of lots, that is, a series long enough to allow the switching rules (Clause 6) to be applied. These switching rules provide:
a) 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;
b) an incentive (at the discretion of the responsible authority) to reduce inspection costs (by means of a switch to reduced inspection) should consistently good quality be achieved.
1.4 This part of ISO 3951 is designed for use under the following conditions:
a) 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. If there are different producers or production
processes, this part of ISO 3951 shall be applied to each one separately;
b) where only a single quality characteristic x of these products is taken into consideration, which must be measurable on a continuous scale;
c) where the uncertainty of the measurement system is negligible with respect to the production process standard deviation;
d) 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) or a distribution
which may be mathematically transformed to a normal distribution;