BS 06002-4:2006 pdf download
BS 06002-4:2006 pdf download.Part 4: Sequential sampling plans indexed by acceptance quality limit(AQL) for inspection by variables(known standard deviation).
7 Planning
The choice of the most suitable variables plan, If one exists, requires experience. judgemeni and some knowledge both of statistics and the product to be inspected. Clauses 8 and 9 are intended to help those responsible for specifying sampling plans in making this choice. They suggest the considerations that should be borne in mend when deciding whether a variables plan would be suitable, and the choices to be made when selecting an appropriate standard plan.
8 Choice between variables and attributes
The first question to consider is whether it is desirable to inspect by variables rather than by attributes 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 elaborate procedure required by a variables scheme, which is usually more time-consuming and costly per Item.
b) In terms of the knowledge gained, the advantage lies with inspection by variables, as the information obtained indicates more precisely how good the product is. Earlier warning wl therefore be given if the quality is slippmg.
C) An attributes scheme can be more readily understood and accepfed: for example, it may at first be difficult 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 variabIes
e) 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 complicated to operate as the number of measurements to be taken on each item increases.
g) The use of this part 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 consulted.
NOTE I ISO 5479 gives detailed procedures for tests for depailure from normality.
NOTE 2 Departure from normality is also dealt with in Clause 2 of ISO 2854:1976, which provides examples of giaphicet methods wluid cai be used 10 verIfy that the disinbution of the data is sufficiently normal to justify the use of samplrig by variables.
9 Choice of Inspection level and AQL
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 and the consequences of nonconforming Items passing Into service. For a standard sampling plan, the inspection level In conjunction 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 b. used for the evaluation of Involved risks.
The normal practice is to use general inspection level II, unless special circumstances indicate that another level is more appropriate
11.2 Sample size code letters
Sample sizes are designated by sample size code letters. Table 3 shall be used to find the applicable code lefler for the particuW lot size and the prescribed inspection level.
NOTE For ecoromy of space m the tables or to avoid unnecessary repetition in the text, the abbreviated term code letter is sometimes used
11.3 Obtaining a sampling plan
The AOL and the sample size code letter shall be used to obtain the sampling plan from Tables Al, A.2 or A.3. For a specified AOL and a given lot size, the same combination of AOL and sample size code letter shall be used to obtain the sampling plan from the table for normal, tightened and reduced inspection.
When no sampling plan is avadable for a given combination of AQL and sample size code letter, arrows in the tables direct the user to a different code letter. The sampling plan to be used is given by the new sample size code letter, not by the original letter. If this procedure leads to different curtailment values for different classes of nonconforming items, the sample size code letter corresponding to the largest curtailment value derived may be used for all classes of nonconforming items, when designated or approved by the responsible authority.
For some combinations of AOL and sample size code letter, the entry in the table Is a star () indicating that the decision cannot be made before the cumulative sample size reaches the sample size of the corresponding single sampling plan. In such a case, the sequential sampling plan does not have any advantage over the single sampling plan, and the user is advised to use the simpler single sampling plan of ISO 3951-I in place of the more complicated sequential sampling plan.
