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# Explain and apply the concepts of random variables

Given a random experiment, with sample space, $$S$$, we can define the possible values of $$S$$ as a random variable. Random variables can be discrete or continuous.

## Discrete Random Variables

A discrete random variable is a variable whose range of possible values is from a specified list. In other words,  a discrete take a countable number of positive values in the sample space.

For a discrete variable $$X$$, we define the probability mass function (PMF) as a function that describes the relationship between a random variable, $$X$$, and the probability of each value of $$X$$ occurring. PMF is mathematically defined as:

$$f\left(a\right)=P(X=a)$$

A probability density function (PDF), sometimes called the probability mass function if the variable is discrete, is a function that describes the relationship between a random variable, $$X$$, and the probability of each value of $$x$$ occurring.

The probability mass function is usually presented graphically by plotting $$f(x_i)$$ on the y-axis against $$x_i$$ on the x-axis. For example, the probability massdensity function shown in the graph below can be written as:

$$f\left( x \right) =\begin{cases} 0.2, & x=1,4 \\ 0.3, & x=2,3 \end{cases}$$

The probability density (or mass) function of a discrete random variable has the following properties:

1. $$f \left(x \right) > 0,\quad x € S;\quad$$ meaning all individual probabilities must be greater than zero.
2. $${ \Sigma }_{ x\in S }f\left( x \right)=1; \quad$$ meaning the sum of all individual probabilities must equal one.
3. $$P\left( X\in A \right) ={ \Sigma }_{ x\in A }f\left( x \right) \quad$$,where $$A\subset S;\quad$$ meaning the probability of event $$A$$ is the sum of the probabilities of the values of $$X$$ in $$A$$.

## Cumulative Distribution Function for Discrete Random Variables

If we are interested in cumulative probabilities then we can use the cumulative distribution function (CDF) which is defined by the function shown below:

$$F\left( x \right) =P\left( X\le x \right) ,\quad \quad -\infty <x<\infty$$

The cumulative distribution function $$F$$ can be expressed in terms of $$f(a)$$ as:

$$F\left(a\right)=\sum_{\forall\ x\le a}{f(x)}$$

For a discrete random variable X whose possible values are $$x_1,x_2,x_3,\ldots$$ for all $$x_1<x_2<x_3<\ldots$$, the cumulative distribution function $$F$$ of $$X$$ is a step function. As such, $$F$$ is constant in the interval $$[x_{i-1},\ x_i]$$ and take a lip (jump) of $$f(x_i)$$ at $$x_i$$.

#### Example: Graph of Cumulative Distribution Function

Consider the following probability distribution:

$$f(x)=\begin{cases}0.2,&x=1,4\\ 0.3, & x=2,3\\\end{cases}$$

Compute the cumulative distribution and plot its graph.

Solution

Consider the following table:

$$\begin{array}{c|c|c|c} \bf{X_i} & \bf{f(x_i)} & \bf{F(x_i)} \\ \hline 1 & 0.2 & 0.2 \\ \hline 2 & 0.3 & 0.5 \\ \hline 3 & 0.3 & 0.8 \\ \hline 4 & 0.2 & 1.0 \\ \end{array}$$

The cumulative distribution function $$F(x_i)$$ can be written as:

$$F\left( x \right) =\begin{cases} 0, & x<1 \\ 0.2, & 1\le x<2 \\ 0.5, & 2\le x<3 \\ 0.8, & 3\le x<4 \\ 1, & x\ge 4 \end{cases}$$

The graph of the cumulative distribution function is as shown below:

## Continuous Random Variable

A continuous random variable is a random variable that has an infinite number of possible values. For a continuous random variable $$X$$, we define a non-negative function $$f(x)$$ for all $$x\in(-\infty,\infty)$$ with the property that for all real values, $$R$$, so that:

$$\Pr{\left(X\in R\right)}=\int_{R}{f(x)}\ dx$$

The probability density function, $$f(x)$$, of a continuous random variable is a differential equation with the following properties:

1. $$f\left( x \right)\ge 0$$: the function (all probabilities) is always greater than zero.
2. $$\int _{ -\infty }^{ \infty }{ f\left( x \right)dx=1 }$$: the sum of all the possible values (probabilities) is one.

It is important to note that unlike the probability density function of a discrete random variable, the probability density function of a continuous random variable is not a probability. In order to calculate a probability using a probability density function, the following function must be used:

$$P\left( X < a \right) =P\left( X\le a \right) =F\left( a \right) =\int _{ -\infty }^{ a }{ f\left( x \right) dx }$$

Also note that, for $$R=\left[a,b\right]$$

$$P\left( X=a \right) =\int _{ a }^{ a }{ f\left( x \right) dx=0 }$$

The graph  of  $$P\left(a\le X\le b\right)$$ if as shown below:

Also, it is important to note that the probability of any individual value of a probability density function is zero, as shown in the formula below:

$$P\left(X=a\right)=\ \int_{a}^{a}{f\left(x\right)}\ dx=0$$

#### Example: Calculating Probability from a PDF

Given the following probability density function of a continuous random variable:

$$f\left( x \right) =\begin{cases} { x }^{ 2 }+c, & 0 < x < 1 \\ 0, & otherwise \end{cases}$$

1. Calculate C.

Solution

\begin{align*}& \int _{ 0 }^{ 1 }{ \left( { x }^{ 2 }+C \right) dx=1 } \\& { \left[ \frac { { x }^{ 3 } }{ 3 } +Cx \right] }_{ x=0 }^{ x=1 }=1 \\& {1}/{3} + C = 1 \\& C = {2}/{3} \\\end{align*}

2. Calculate $$P (X > {\frac {1}{2})}$$.

Solution

\begin{align*}& \int _{ { {1}/{2}} }^{ 1 }{ \left( { x }^{ 2 }+{ {2}/{3}} \right) dx } \\& { \left[ \frac { { x }^{ 3 } }{ 3 } +{ {2}/{3}}x \right] }_{ x={\frac {1}{2}} }^{ x=1 } \\& \left[ {1}/{3}+{2}/{3} \right] – \left[{1}/{24}+{1}/{3} \right] = {5}/{8} \\\end{align*}

## Conditional Probability

Conditional probabilities of random variables can be calculated in a similar way to empirical conditional probabilities, as shown in the formula below:

$$P\left( A|B \right) =\frac { P\left( A\cap B \right) }{ P\left( B \right) }$$

Let event $$A$$ be $$X > 2$$ and event $$B$$ be $$X < 4$$ then

$$P\left( A|B \right) =\frac { P\left( 2<X<4 \right) }{ P\left( X<4 \right) }$$

#### Example: Conditional Probability

Given the following probability density function of a continuous random variable:

$$f\left( x \right) =\begin{cases} { x }^{ 2 }+{2}/{3}, & 0 < x < 1 \\ 0, & otherwise \end{cases}$$

Calculate $${P} \left({ {X} }>\cfrac{3}{4}∣{ {X} }>\cfrac{1}{2}\right)$$

#### Solution

Using the fact that:

$$P\left(A\mid B\right)=\frac{P\left(A\cap B\right)}{P(B)}$$

We have,

\begin{align} P(X>\frac{3}{4}\mid X>\frac{1}{2})\ &=\ \frac{P(X>\frac{3}{4}\cap X>\frac{1}{2})\ }{P(X>\frac{1}{2})}\\ &=\frac{P\left(X>\frac{3}{4}\right)}{P(X > \frac{1}{2})}\end{align}

Now.

\begin{align*} P\left(X>\frac{3}{4}\right) & =\int _{ { \frac{3}{4} } }^{ 1}{\left({ {x} }^{2}+\cfrac{2}{3}\right) {dx} } \\ & = 0.359375 \\ {P}\left({ {X} }>\cfrac{1}{2}\right) & =\int _{ { \frac{1}{2} } }^{ 1 }{ \left({ {x} }^{2}+\cfrac{2}{3}\right) {dx} }=\left[ \cfrac{ { {x} }^{3} }{3}+\cfrac{2}{3} { {x} } \right]_{{ {x} }=\frac{1}{2}}^{{ {x} } = 1} =\cfrac{5}{8} \\ \Rightarrow {P} \left( { {X} } > \cfrac{3}{4}|{ {X} }>\cfrac{1}{2}\right) &= \cfrac{0.359375}{\cfrac{5}{8}} =0.575 \end{align*}

Learning Outcome

Topic 2.a – b: Univariate Random Variables – Explain and apply the concepts of random variables, probability and probability density functions, cumulative distribution functions & Calculate conditional probabilities.

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