ISO 3951-3:2007 Sampling procedures for inspection by variables - Part 3: Double sampling schemes indexed by acceptance quality limit (AQL) for lot-by-lot inspection
International Organization for Standardization (ISO)
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ISO 3951-3:2007
Scope
This part of ISO 3951 specifies an acceptance sampling system of double sampling schemes for inspection by
variables for percent nonconforming. It is indexed in terms of the acceptance quality limit (AQL).
The objectives of the methods laid down in this part of ISO 3951 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 this part of ISO 3951, 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.
This part of ISO 3951 is primarily 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, apply this part of ISO 3951 to each one separately;
b) where the items of product have a single quality characteristic (for multiple quality characteristics, see
informative Annexes A, B and C);
c) where the quality characteristic is measurable on a continuous scale;
d) where the measurement error is negligible (i.e. with a standard deviation of no more than 10 % of the
corresponding process standard deviation);
e) where production is stable (under statistical control) and the quality characteristic is distributed, at least to
a close approximation, according to a normal distribution;
CAUTION — The procedures in this part of ISO 3951 are not suitable for application to lots that have
been screened previously for nonconforming items.
f) where the possibility of having to select and inspect a second sample is administratively acceptable;
ISO 3951-3:2007 Referenced Document
ISO 3534-1 Statistics — Vocabulary and symbols — Part 1: General statistical terms and terms used in probability
ISO 3534-2:2006 Statistics - Vocabulary and symbols - Part 2: Applied statistics
ISO 3951-3:2007 history
2007ISO 3951-3:2007 Sampling procedures for inspection by variables - Part 3: Double sampling schemes indexed by acceptance quality limit (AQL) for lot-by-lot inspection