162 CHAPTER 5 JOINT PROBABILITY DISTRIBUTIONS
We have obtained the marginal probability density function of Y. Now,
5-3.3 Conditional Probability Distributions
Analogous to discrete random variables, we can define the conditional probability distribution
of Ygiven Xx.
610 ^3 c
e^4
0.002
e^6
0.003
d0.05
610 ^3 ca
e0.002y
0.002
`
2000
ba
e0.003y
0.003
`
2000
bd
P 1 Y
20002 6 10 ^3
2000
e0.002y 11 e0.001y 2 dy
Given continuous random variables Xand Ywith joint probability density function
fXY(x, y), the conditional probability density function of Ygiven Xxis
fY |x 1 y 2 (5-18)
fXY 1 x, y 2
fX 1 x 2
for fX 1 x 2
0
Definition
The function fY|x(y) is used to find the probabilities of the possible values for Ygiven
that X x. Let Rxdenote the set of all points in the range of (X, Y) for which X x. The
conditional probability density function provides the conditional probabilities for the values
of Yin the set Rx.
Because the conditional probability density function is a probability density
function for all yin Rx, the following properties are satisfied:
(1)
(2)
(3)
(5-19)
P 1 YB 0 Xx 2
B
fY 0 x 1 y 2 dy for any set B in the range of Y
Rx
fY 0 x 1 y 2 dy 1
fY (^0) x 1 y 2 0
fY | x 1 y 2
It is important to state the region in which a joint, marginal, or conditional probability
density function is not zero. The following example illustrates this.
EXAMPLE 5-17 For the random variables that denote times in Example 5-15, determine the conditional prob-
ability density function for Ygiven that Xx.
First the marginal density function of xis determined. For x
0
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