Applied Statistics and Probability for Engineers

18 CHAPTER 2 PROBABILITY

uncontrollable inputs, the same settings for the controllable inputs do not result in identical

outputs every time the system is measured.

Voltage

Current

Figure 2-3 A closer examination of the system

identifies deviations from the model.

0 5101520

1234

Minutes

Call

Call duration

Time

0 5101520

123

Minutes

Call

Call duration

Time

Call 3 blocked

Figure 2-4 Variation causes disruptions in the system.

An experiment that can result in different outcomes, even though it is repeated in the

same manner every time, is called a random experiment.

Definition

For the example of measuring current in a copper wire, our model for the system might

simply be Ohm’s law. Because of uncontrollable inputs, variations in measurements of current

are expected. Ohm’s law might be a suitable approximation. However, if the variations are

large relative to the intended use of the device under study, we might need to extend our model

to include the variation. See Fig. 2-3.

As another example, in the design of a communication system, such as a computer or

voice communication network, the information capacity available to service individuals using

the network is an important design consideration. For voice communication, sufficient

external lines need to be purchased from the phone company to meet the requirements of a

business. Assuming each line can carry only a single conversation, how many lines should be

purchased? If too few lines are purchased, calls can be delayed or lost. The purchase of too

many lines increases costs. Increasingly, design and product development is required to meet

customer requirements at a competitive cost.

In the design of the voice communication system, a model is needed for the number of calls

and the duration of calls. Even knowing that on average, calls occur every five minutes and that

they last five minutes is not sufficient. If calls arrived precisely at five-minute intervals and lasted

for precisely five minutes, one phone line would be sufficient. However, the slightest variation in

call number or duration would result in some calls being blocked by others. See Fig. 2-4. A system

designed without considering variation will be woefully inadequate for practical use. Our model

for the number and duration of calls needs to include variation as an integral component. An

analysis of models including variation is important for the design of the phone system.

2-1.2 Sample Spaces

To model and analyze a random experiment, we must understand the set of possible out-

comesfrom the experiment. In this introduction to probability, we make use of the basic

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