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概率论与数理统计英文版3精选
3. Random Variables
3.1 Definition of Random Variables
In engineering or scientific problems, we are not only interested in
the probability of events, but also interested in some variables depending
on sample points. (定义在样本点上的变量)
For example, we maybe interested in the life of bulbs produced by a
certain company, or the weight of cows in a certain farm, etc. These ideas
lead to the definition of random variables.
1. random variable definition
Definition 3.1.1 A random variable is a real valued function defined on
a sample space; i.e. it assigns a real number to each sample point in the
sample space.
Here are some examples.
Example 3.1.1 A fair die is tossed. The number X shown is a random
variable, it takes values in the set {1,2,6}.
Example 3.1.2 The life t of a bulb selected at random from bulbs
produced by company A is a random variable, it takes values in the
interval (0 , ∞) .
Since the outcomes of a random experiment can not be predicted in
advance, the exact value of a random variable can not be predicted before
the experiment, we can only discuss the probability that it takes some
27
value or the values in some subset of R.
2. Distribution function
Definition 3.1.2 Let X be a random variable on the sample space S .
Then the function
F (X ) P (X =≤x ) . x ∈R
is called the distribution function of X
Note The distribution function F (X ) is defined on real numbers, not on
sample space.
Example 3.1.3 Let X be the number we get from tossing a fair die.
Then the distribution function of X is (Figure
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