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152 Part III: Inspection and Test
SAMPLING CONCEPTS
Sampling may be performed according to the type of quality characteristics to be
inspected. There are three major categories of sampling plans: sampling plans for
attributes, sampling plans for variables, and special sampling plans.
Random Sampling
The simple random sampling design consists of selecting n (sample size) sampling
units in such a way that each sampling unit has the same chance of being
selected.
The Operating Characteristic Curve
Part III.B
No matter which type of attribute sampling plan is being considered, the most
important evaluation tool is the operating characteristic (OC) curve.
The OC curve allows a sampling plan to be almost completely evaluated at
a glance, giving a pictorial view of the probabilities of accepting lots submitted
at varying levels of percent defective. The OC curve illustrates the risks involved
in acceptance sampling. Figure 15.1 shows an OC curve for a sample size n of 50
drawn from an infinite lot size, with an acceptance number c of 3.
As can be seen by the OC curve, if the lot were 100 percent to specifications,
the probability of acceptance P a also would be 100 percent. But if the lot were 13.4
percent defective, there would be a 10 percent probability of acceptance.
1.00
0.90
0.80
0.70
n = 50
c = 3
P a
0.60
0.50
0.40
0.30
0.20
0.10
0
1 2 3 4 5 6 7 8 9 10 15
p (percent nonconforming)
20
Figure 15.1 An operating characteristic (OC) curve.