ABC Standard (Metrology)

19.28.
ABC standard is based on AQL (“Acceptable Quality Level”) which is a point on the OC curve at which a batch is very likely to be accepted. The probability of a lot or batch containing exactly the AQL percentage of defectives being accepted is often set at 0.95 but other values near this are sometimes used.
Many plans based on AQL have been developed and revised from time to time. The latest development was in the year 1963, when United States adopted a plan known as MIL-STD-105D. The international designation of MIL-STD-105D is ABC-STD-105.
ABC Standard Tables have been developed which give sample size and acceptance and rejection numbers. To enter any one of these tables AQL and sample size code letter must be known. To determine which table should be used, it is necessary to know whether single, double or multiple sampling is to be used and to know whether normal, tightened or reduced inspection is to be used.
The AQL values in the standard may be interpreted either as defects per hundred units or as per cent defective depending whether acceptance criteria are to be based on the number of defects or the number of defectives observed in a sample. However, AQL values above 100 are interpreted as applying to defects per hundred units.
All the AQL values are multiples of the numbers 1,1.5,2.5,4.0 and 6.5. These numbers are roughly in a geometrical progression and correspond to systems of “preferred numbers” in common use for other industrial purposes.
There is a table which relates the lot or batch size and sample size code letter that determines the sample size. It assumes three general inspection levels which are ones used in most cases. The Standard states: “Unless otherwise specified, Inspection Level II will be used. However, Inspection Level I may be specified when less discrimination is needed, or Level III may be specified for greater discrimination”.
It also assumes four special levels S-l to S-4 which are to be used for the special case where relatively small sample sizes are necessary and larger sampling risks can or must be tolerated.
In ABC standard, AQL is defined as the maximum per cent defective (or the maximum number of defects per hundred units) that, for purpose of sampling inspection, can be considered satisfactory as a process average.
In all AQL systems, the acceptance criteria under normal inspection have been chosen to protect the producer against rejection of lots meeting the quality standard. However, in most AQL systems the producer’s risk that such lots will be rejected is not the same for all sampling plans. It depends on the sample size-’smaller the sample size, more the risk the producer takes that a lot will be rejected when it contains defectives exactly-equal to AQL% with a smaller sample size, the consumer’s risk of accepting a lot much worse than the AQL is also much greater.
All the military AQL systems have used a somewhat empirical relationship between lot size and sample size. Unless a sample is a substantial fraction of its lot, the OC curve of the sampling plan is practically independent of lot size, depending almost entirely on sample size and acceptance number. Although the absolute sample size increases with lot size in the military AQL system, the relative sample decreases.
In ABC standard, the sample size code letter depends only on the lot size and inspection level and is not influenced by the AQL. The military standards have not provided an automatic decrease in sample size with a relation of the quality standard, as in the case of Philips Standard sampling system. In ABC standard, the sample size is further determined by the AQL and the sample size code letter

Criteria for Shifting to Tightened Inspection and Requalification for normal Inspection.
ABC standard states : “When normal inspection is in effect, tightened inspection shall be instituted when 2 out of 5 consecutive lots or batches have been rejected on original inspection (i.e. ignoring resubmitted lots or batches for this procedure”.
The statement regarding requalification for normal inspection is as follows : “when tightened inspection is in effect, normal inspection shall be instituted when five consecutive lots or batches have been considered acceptable on original impaction”.
Criteria for Qualification and loss of Qualification for Reduced Inspection. The ABC standard states the following conditions for a shift from normal to reduced inspection :
(a) The preceding 10 lots or batches (or more as indicated in relevant table have been on normal inspection and none has been rejected on original inspection ; and
(b) The total number of defective (or defects) in the samples from the preceding 10 lots or batches (or such other number as was used for condition (a) above is equal to or less than the applicable number given in the relevant table. If double or multiple sampling is in use, all samples inspected should be included, not “first” samples only ; and
(c) Production is at a steady rate ; and
(d) Reduced inspection is considered desirable by the responsible authority.
Normal inspection must be reinstated whenever “a lot or batch is rejected” or “production becomes irregular or delayed, or other conditions warrant that normal inspection shall be instituted.