4.7 Switching Procedures.
4.7.1 Normal to Tightened.
When normal inspection is in effect, tightened inspection shall be instituted when 2
out of 2, 3, 4, or 5 consecutive lots or batches have been rejected on original inspection (i.e., ignoring
resubmitted lots or batches for this procedure).
4.7.2 Tightened to Normal.
When tightened inspection is in effect, normal inspection shall be instituted when 5
consecutive lots or batches have been considered acceptable on orignial inspection.
4.7.3 Normal to reduced.
When normal inspection is in effect,reduced inspection shall be instituted provided that
all of the following conditions are satified:
a. The preceding 10 lots orbatches (or more ,as indicated by the note to Table VIII) have been on normal
inspection and all have been accepted on original inspection; and
b. The total number of defectives (or defects) in the samples form 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 Table
VIII. If double or multiple sampling is in use,all samples inspected should be included, not "first" samples
c. Production is at a steady rate; and
d.Reduced inspection is considered desirable.
4.7.4 Reduced to Normal.
When reduced inspection is in effect, normal inspection shall be instituted if any of
the following occur on original inspection:
a. A lot or batch is rejected; or
b. A lot or batch is considered acceptable under the procedures of 18.104.22.168, or
c. Production becomes irregular or delayed; or
d. Other conditions warrant that normal inspection shall be instituted.
4.8 Descontinuation of Inspection.
If the cummulative number of lots not accepted in a swquence of consecutive
lots on original tightened inspection reaches five, the acceptance procedures of this standard shall be
discontinued. inspection under the provisions of this standard shall not be resumed until corrective action has
been taken. Tigthened inspection shall the be used as if 4.7.1 had been invoked.
4.9 Sampling plans
4.9.1 Inspection Level.
The inspection level determines the relationship between the lot or batch size and the
sample size. The inspection level to be used for any particular requirement will be as prescribed by the
contractor's written procedures. Three inspection levels: I, II, and III, are given in Table I for general use
(see 4.1). Normally, Inspection Level II is used. However, Inspection Level I may be used when less discriminatin
is needed, or Level III may be used for greater discrimination. Four additiaonal special levels: S-1, S-2, S-3,
and S-4, are given in the same table and may be used where relatively small sample sizes are necessary and large
sampling risks can or must be tolerated.
NOTE: In the selection of inspection levels S-1 to S-4, care must be exercised avoid AQLs inconsistent with these
inspection levels. In other words, the purpose of the special inspection levels is to keep samples small when
necessary. For instance, the code letters under S-1 go no further than D, equivalent to a single sample of size
8, but it is of no use to choose S-1 if the AQL is 0.10 percent which the minimum sample is 125.
4.9.2 Code Letters.
Sample size are designated by code letters. Table I shall be used to find the applicable
code letter for the particular lot or batch size and the prescribed inspection level.
4.9.3 Obtaining Sampling Plan.
The AQL and the code letter shall be used to obtain the samplling plan from Tables
II. III, or IV. When no sampling plan is available for a given combination of AQL and code letter,the tables
direct the use to a different letter. The sample size to be used is given by the new code letter, not by the
original letter. if this procedure leads to different sample sizes for different classes of defects, the code
letter corresponding to the largest sample size derived may be used for all classes of defects. As an alternative
to a single sampling plan with an acceptance number of 0, the plan with an acceptance number of 1 with its
correspondingly larger sample size for a designated AQL (where available), may be used.
4.9.4 Types of Sampling Plans.
Three types of sampling plans: single,Double, and Multiple, are given in Tables
II, III, and IV, respectively. When several types of plans are available for a given AQL and code letter, any one
may be used.A decision as to type of plan, either single, double, or multiple, when available for a given AQL and
code letter, will usually be based upon the comparision between the administrative difficulty and the average
smaple sizes of the available plans. The average sample size of multiple plans is less than for double (except in
the case corresponding to single acceptance number 1) and both of these are always less than a single sample size
(see Table IX). Usually the administrative difficulty for single sampling and the cost per unit of the sample are
less than for double or multiple.