Applied Statistics and Probability for Engineers

As in Example 3-16, equals the total number of different sequences of trials that

contain xsuccesses and nxfailures. The total number of different sequences that contain x

successes and nxfailures times the probability of each sequence equals

The probability expression above is a very useful formula that can be applied in a num-

ber of examples. The name of the distribution is obtained from the binomial expansion. For

constants aand b, the binomial expansion is

Let pdenote the probability of success on a single trial. Then, by using the binomial

expansion with a  pand b 1 p, we see that the sum of the probabilities for a bino-

mial random variable is 1. Furthermore, because each trial in the experiment is classified

into two outcomes, {success, failure}, the distribution is called a “bi’’-nomial. A more

1 a b 2 n a

n

k 0

a

n

k

b akbnk

P 1 Xx 2.

a

n

x

b

74 CHAPTER 3 DISCRETE RANDOM VARIABLES AND PROBABILITY DISTRIBUTIONS

A random experiment consists of nBernoulli trials such that

(1) The trials are independent

(2) Each trial results in only two possible outcomes, labeled as “success’’ and

“failure’’

(3) The probability of a success in each trial, denoted as p, remains constant

The random variable Xthat equals the number of trials that result in a success

has a binomial random variablewith parameters and The

probability mass function of Xis

f 1 x 2 a (3-7)

n

x

b px 11 p 2 nx x0, 1,p, n

0 p 1 n1, 2,p.

Definition

To complete a general probability formula, only an expression for the number of outcomes

that contain xerrors is needed. An outcome that contains xerrors can be constructed by parti-

tioning the four trials (letters) in the outcome into two groups. One group is of size xand

contains the errors, and the other group is of size n  xand consists of the trials that are okay.

The number of ways of partitioning four objects into two groups, one of which is of size x, is

. Therefore, in this example

Notice that , as found above. The probability mass function of X

was shown in Example 3-4 and Fig. 3-1.

The previous example motivates the following result.

a

4

2

b 4 ! 32! 2! 4  6

P 1 Xx 2 a

4

x

b 1 0.1 2 x 1 0.9 24 x

a

4

x

b

4!

x! 14 x 2!

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