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Chapter 15: B. Sampling 151
units produced to date conform to the quality standards agreed upon
with the customer.
6. To rate the accuracy of inspectors or of inspection equipment by comparing the
inspection results with corresponding standards. An inspection operation
can result in two types of error: classification of a conforming unit as
nonconforming or classification of a nonconforming unit as conforming.
The probabilities of both types of error can be easily estimated using
probability theory and other statistical methods.
7. To serve as a mechanism for evaluating vendors in terms of their products’
quality. Vendors that consistently deliver high-quality products can
receive preferred status involving reduced inspection and priority in
bidding for new contracts, while vendors that do not stand up to quality
requirements could be warned or discontinued altogether. This type of
procedure is known as vendor qualification or vendor certification.
ADVANTAGES OF SAMPLING
Sampling provides the economic advantage of lower inspection costs due to fewer
units being inspected. In addition, the time required to inspect a sample is substantially
less than that required for the entire lot and there is less damage to the
product due to reduced handling. Most inspectors find that selection and inspection
of a random sample is less tedious and monotonous than inspection of a complete
lot. Another advantage of sampling inspection is related to the supplier/
customer relationship. By inspecting a small fraction of the lot and forcing the
supplier to screen 100 percent in case of lot rejection (which is the case for rectifying
inspection), the customer emphasizes that the supplier must be concerned
about quality. On the other hand, the variability inherent in sampling results in
sampling errors: rejection of lots of conforming quality and acceptance of lots of
nonconforming quality.
Acceptance sampling is most appropriate when inspection costs are high and
when 100 percent inspection is monotonous and can cause inspector fatigue and
boredom, resulting in degraded performance and increased error rates. Obviously,
sampling is the only choice available in destructive inspection. Rectifying sampling
is a form of acceptance sampling. Sample units detected as nonconforming
are discarded from the lot, replaced with conforming units, or repaired. Rejected
lots are subject to 100 percent screening, which can involve discarding, replacing,
or repairing units detected as nonconforming.
In certain situations, it is preferable to inspect 100 percent of the product.
This would be the case for critical or complex products, where the cost of making
the wrong decision would be too high. Screening is appropriate when the fraction
nonconforming is extremely high. In this case, most of the lots would be rejected
under acceptance sampling and those accepted would be so as a result of statistical
variations rather than better quality. Screening is also appropriate when the
fraction nonconforming is not known and an estimate based on a large sample
is needed.
Part III.B