BS ISO 3951-2:2013

This standard BS ISO 3951-2:2013 Sampling procedures for inspection by variables is classified in these ICS categories:

• 03.120.30 Application of statistical methods

This part of ISO 3951 is primarily designed for use under the following conditions:

1. 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;

2. where the quality characteristics of the items of product are measurable on a continuous scale;

3. where the measurement error is negligible (i.e. with a standard deviation no more than 10 % of the corresponding process standard deviation). However, procedures are also provided in Clause 9 and Annex P for accommodating measurement error when it has a non-negligible standard deviation;

4. where production is stable (under statistical control) and the quality characteristics are distributed, at least to a close approximation, according to normal distributions;

5. where, in the case of multiple quality characteristics, the characteristics are independent, or almost independent, of one another;

6. where a contract or standard defines a lower specification limit, L, an upper specification limit, U, 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. 1)

      x ≥ L (i.e. the lower specification limit is not violated);

   2. 2)

      x ≤ U (i.e. the upper specification limit is not violated);

   3. 3)

      x ≥ L and x ≤ U (i.e. neither the lower nor the upper specification limit is violated).

7. If there are two or more, say m, quality characteristics, then, designating the lower and upper limits for the i^th quality characteristic by L_i and U_i respectively, an item of product is qualified as nonconforming if one or more of its m measured quality characteristics, x_i, fails to satisfy the appropriate one of the following inequalities:

   1. 4)

      x_i ≥ L_i;

   2. 5)

      x_i ≤ U_i;

   3. 6)

      x_i ≥ L_i and x_i ≤ U_i.

Inequalities 1), 2), 4), and 5) are called cases with a single specification limit while 3) and 6) are called cases with double specification limits. For double specification limits, a further distinction is made between combined control, separate control, and 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, and

• 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.

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.