Sampling procedures for inspection by variables - General guide to single sampling plans indexed by acceptance quality limit (AQL) for lot-by-lot inspection of independent quality characteristics
This British Standard describes an acceptance sampling system
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;
b) where the quality characteristics of the items of product are
measurable on a continuous scale;
c) where the measurement error is negligible (i.e. with a standard
deviation no more than 10% of the corresponding process
standard deviation);
d) where production is stable (under statistical control) and the
quality characteristics are distributed, at least to a close
approximation, according to normal distributions;
e) where, in the case of multiple quality characteristics, the
characteristics are independent, or almost independent, of one
another;
f) where a contract or standard defines a lower specification limit
U, an upper specification limit L, or both on each of the quality
characteristics. If there is only one quality characteristic, an item
is qualified as conforming if its measured quality characteristic x
satisfies the appropriate one of the following inequalities:
1) x W L (i.e. the lower specification limit is not violated);
2) x u U (i.e. the upper specification limit is not violated);
3) x W L and x u U (i.e. neither the lower nor the upper specification limit is violated).
If there are two or more, say m, quality characteristics, then,
designating the lower and upper limits for the ith quality characteristic by Li and Ui respectively, an item of product is qualified as nonconforming if one or more of its m measured quality characteristics xi fails to satisfy the appropriate one of the following inequalities:
4) xi W Li;
5) xi u Ui;
6) xi W Li and xi u Ui.
Inequalities 1), 2), 4) and 5) are called cases with a single
specification limit, and 3) and 6) are called cases with double
specification limits. For double specification limits, a further
distinction is made between combined control, separate control or
complex control. If there is only one quality characteristic, then:
? combined control is where a single AQL applies to nonconformity
beyond both limits;
? separate control is where separate AQLs apply to nonconformity
beyond each of the limits;
? complex control is where one AQL applies to nonconformity
beyond the limit that is of greater seriousness, and a larger AQL
applies to the total nonconformity beyond both limits.
Licensed Copy: Wang Bin, na, Tue Apr 17 02:03:41 GMT+00:00 2007, Uncontrolled Copy, (c) BSI BS 6002-2:2007 6 ? ? BSI 2007
If there are two or more quality characteristics, this generalizes as
follows:
? combined control is where nonconformity beyond both limits on a
variable belongs to the same class, to which a single AQL applies;
? separate control is where nonconformity beyond the two limits on
a variable belongs to separate classes, to each of which a single
AQL applies;
? complex control is where nonconformity beyond the limit that is of
greater seriousness belongs to one class to which a single AQL
applies, and the total nonconformity beyond both limits belongs to
another class to which a larger AQL applies.
Note that, in the case of two or more quality characteristics,
nonconformity on more than one quality characteristic may belong to the same class