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Chapter 15: B. Sampling 155
TYPES OF ATTRIBUTE SAMPLING PLANS
There are several types of attribute sampling plans in use, with the most common
being single, double, multiple, and sequential sampling plans. The type of sampling
plan used is determined by ease of use and administration, general quality
level of incoming lots, average sample number, and so on.
Single Sampling Plans
When single sampling plans are used, the decision to either accept or reject the
lot is based on the results of the inspection of a single sample of n items from a
submitted lot. In Figure 15.2, the OC curve and AOQ curve were calculated for
a single sampling plan where n = 50 and c = 3. Single sampling plans have the
advantage of ease of administration, but due to the unchanging sample size, they
do not take advantage of the potential cost savings of reduced inspection when
incoming quality is either excellent or poor.
Double Sampling Plans
When using double sampling plans, a smaller first sample is taken from the submitted
lot, and one of three decisions is made: 1) accept the lot, 2) reject the lot,
or 3) draw another sample. If a second sample is to be drawn, the lot will either
be accepted or rejected after the second sample. Double sampling plans have the
advantage of a lower total sample size when the incoming quality is either excellent
or poor because the lot is either accepted or rejected on the first sample.
Part III.B
Multiple Sampling Plans
Multiple sampling plans work in the same way as double sampling with an extension
of the number of samples to be taken up to seven, according to ANSI/ASQ
Z1.4-2003. In the same manner that double sampling is performed, acceptance or
rejection of submitted lots may be reached before the seventh sample, depending
on the acceptance/rejection criteria established for the plan.
SAMPLING STANDARDS AND PLANS
ANSI/ASQ Z1.4-2003
ANSI/ASQ Z1.4-2003 is probably the most commonly used standard for attribute
sampling plans. The wide recognition and acceptance of the plan could be
due to government contracts stipulating the standard, rather than its statistical
importance.
When using ANSI/ASQ Z1.4-2003 the characteristics under consideration
should be classified. The general classifications are critical, major, and minor
defects:
• Critical defect. A critical defect is a defect that judgment and experience
indicate is likely to result in hazardous or unsafe conditions for the