ISO 3951.5 pdf download – Sampling procedures for inspection byvariables — Part 5: Sequential sampling plans indexed byacceptance quality limit (AQL) for inspection by variables (known standarddeviation).
This part of ISO 3951 also provides the poesibdity of switching to reduced inspection when inspection results indicate thai the quality level is stable and reliable at a level better than the AOL This pcectice e. however, optional (at the disuetion of the responsible authoflty).
When It has been necessary to discontinue sampling inspection, inspection shall not be resumed until action has been taken by the producer to improve the quality of the submitted product.
Oetads of the operation of the switching rules and the discontinuation rule are given In 62 and 63
8.2 Standard switching rules
6.2.1 Normal inspection
Normal inspection i3 used at the start of inspection (unless otherwise designated) and shall continue to be used during the course of inspection unti Iitened inspection becomes necessary or reduced inspection is adowed.
62.2 Tlght.rwd Inspection Instituted
Tightened inspection shall be instituted when Iwo lots on original normal inspection are not accepted within
any five or fewer successive lots
6.2,3 Tightened Inspection relaired
Tightened inspection shall be relaiced when five successive lots on original inspection have been accepted on tightened mspection; then normal inspection shall be reinstated.
624 Reduced InspectIon Instituted
Reduced inspection may be instituted after ten successive lots have been accepted under normal inspection, provided that
a) the a.imulatlve sample size for each lot does not exceed O,Sat;
b) production is in statistical control:
c) reduced inspection is considered desirable by the responsible authority.
6.2.5 Reduced inspection ceased and normal inspection reinstated
Reduced inspection shall cease and normal inspection be reinstated if any of the following occ on original inspection
a) a lot is not accepted:
b) production becomes irregular or delayed;
c) reduced inspection is no longer considered desirable by the responsible authority.
6.3 Discontinuation and resumption of inspection
II the cumuiatrve number of lots not accepted in a sequence of consecutive lots on original t.ghtwü Inspection reaches 5, the acceptance procedures of this part of ISO 3951 shall be discontinued
Inspection under the provisions of this part of ISO 3951 shall not be resumed until action has been aken by the supplier to improve the quality of the submitted product or service. Tightened inspection shall then be used as II 62.2 had been invoked,
8 Choice between variables and attributes
The first question to consider is whether It is desirable to inspect by vanables rather than by attnbules. The following points should be taken Into account
a) In terms of economics, it is necessary to compare the total cost of the relatively simple inspection of a larger number of items by means of an attributes scheme with the generally more elabcxate procedure reqed by a variables scheme. wtticti is usuaity more time-oonsurnáng and costly per item.
b) In terms of the knowledge gawied, the advantage bee with inspection by variables, as the information obtained indicates more precisely how good the product is Earlier warning will therefore be given W the quality is slipping.
c) An attributes scheme can be more readily understood and accepted: for example, it may at first be d.ffic.it to accept that when inspecting by variables, a lot can be rejected on measurements taken of a sample that does not contain any nonconforming items.
d) Inspection by variables is particularly appropriate in conjunction with the use of control charts for variables.
a) Variables sampling has a substantial advantage when the inspection process is expensive, for example in the case of destructive testing.
I) A variables scheme becomes relatively more oonipbcated to operate as the number of measurements to be taken on each item increases.
g) The use of this pert of ISO 3951 is only applicable when there is a reason to believe that the distribution of measurements of the quality characteristic is normal. In case of doubt, the responsible authority should be consubed.
NOTE 1 ISO 5479 gives deted procedures lor tests for deartuire from normality.
NOTE 2 Departure from normality i also dealt eie si Clause 2 of ISO 2854:1978, which provides examples cit graphical methods which can ti. used to verity that the isstflbubon of the data ii sufficiently normal to lUISfY the use of sampling by vafliole5
9 Choice of inspection level and AQI
The choice of Inspection level and AOL Is governed by a number of factors, but is mainly a balance between the total cost of Inspection end the consequences 01 nonconforming Items passing into service For a standard sampling plan, the Inspection level In conlunction with the size of the lots and AOl. determines the expected size of the sample to be taken, and governs the seventy of the inspection. The OC curves of equivalent single sampling plans given In ISO 3951-1 (see Annex M of ISO 3951.1:2005) should be used for the evaluation of Involved risks.
ISO 3951.5 pdf download – Sampling procedures for inspection byvariables — Part 5: Sequential sampling plans indexed byacceptance quality limit (AQL) for inspection by variables (known standarddeviation)
