# Discrete Random Variable and Probability Mass Function

Last Updated: 10th October, 2023

Discrete random variables and probability mass functions are essential concepts in probability theory and statistics. In this lesson we will be learning in detail what is discrete random variables and probability mass functions.

A discrete random variable can only take on a countable number of values, and its probability is defined by the probability mass function. Mean and variance provide information about the central tendency and variability of the distribution. These concepts have applications in various fields, but it's important to consider the limitations and challenges associated with their use to ensure accurate results.

## Discrete Random Variables

A random variable may be a variable whose esteem is subject to randomness or uncertainty. A discrete random variable may be a variable that can as it were take on a countable number of values, and each esteem contains a non-zero probability of happening.

For illustration, the number of heads in a coin toss or the number of children in a family are both discrete random variables. The likelihood of each conceivable result of a discrete random variable is characterized by the probability of mass function.

## Probability Mass Function

A probability mass function (PMF) is a function that describes the probability distribution of a discrete random variable. For each possible value that the random variable can take, the PMF gives the probability that the random variable takes that particular value.

Formally, for a discrete random variable X that can take on the values x_1, x_2, ..., x_n, the PMF is defined as:

P(X = x_i) = p_i


where P is the probability function, X is the random variable, and p_i is the probability that X takes on the value x_i.

The PMF has the following properties:

1. For any value x_i, 0 ≤ P(X = x_i) ≤ 1.

2. The sum of all probabilities in the PMF is equal to 1:

ΣP(X = x_i) = 1


3. The PMF can be used to calculate the expected value (or mean) of X:

E(X) = Σx_i * P(X = x_i)


The PMF is a fundamental concept in probability theory and is often used to describe the distribution of outcomes for discrete random variables such as the number of heads in a series of coin tosses, the number of defects in a production line, or the number of cars that pass through an intersection in a given time period.

### Common Discrete Probability Distributions

• There are a few common discrete probability distributions, counting the Bernoulli distribution, Binomial distribution, Poisson distribution, and Geometric distribution.
• The Bernoulli distribution portrays the likelihood of a single trial that can as it were result in victory or disappointment, such as a coin toss.
• The Binomial distribution depicts the likelihood of a series of n independent Bernoulli trials, such as the number of heads in n coin hurls.
• The Poisson distribution depicts the likelihood of the number of occasions happening in a settled interval of time or space, such as the number of mishaps in a day.
• The Geometric distribution depicts the likelihood of the number of trials required to attain the primary success, such as the number of coin hurls required to induce the primary head.

### Mean and Variance of Discrete Random Variables

• The mean and variance of a discrete random variable provide information about the central tendency and variability of the distribution.
• The mean, or expected value, of a discrete random variable X is defined as
E(X) = ΣxP(X = x)


where x is the possible outcome and P(X = x) is the probability of that outcome.

• The variance of a discrete random variable X is defined as
Var(X) = E((X - μ)^2) = Σ(x - μ)^2P(X = x)


where μ is the mean of the distribution.

• The standard deviation of a discrete random variable is the square root of the variance.

### Examples and Applications

• Discrete random variables and probability mass functions are used in various fields such as finance, engineering, and biology.
• For example, in finance, discrete random variables can be used to model stock prices or interest rates, while in engineering, they can be used to model the number of defects in a manufacturing process.
• In biology, discrete random variables can be used to model the number of offspring in a population or the number of bacteria in a culture.

### Limitations and Challenges

• While discrete random variables and probability mass functions are valuable devices in information science, there are restrictions and challenges related to their use.
• Suspicions made about the basic distribution may not continuously hold genuine in real-world applications, and data necessities may be troublesome to meet.
• Moreover, exceptions can significantly affect the mean and variance of a distribution, driving to one-sided comes about.
• It's vital to carefully consider these confinements and challenges when utilizing discrete random variables and probability mass functions in information science applications and to carefully assess the quality and precision of the information being utilized.

## Conclusion

In conclusion, discrete random variables and probability mass functions are important concepts in probability theory and statistics that are used in various fields of data science. By understanding the definition of discrete random variables, the probability mass function, common discrete probability distributions, and how to calculate the mean and variance of a distribution, data scientists can better model and analyze data. However, it's important to also consider the limitations and challenges associated with their use to ensure accurate and reliable results.

## Key Takeaways

1. Discrete random variables are used to model situations where the outcome can only take on a countable number of values.
2. Probability mass function (PMF) defines the probability of each possible value of a discrete random variable.
3. PMF should satisfy two conditions: non-negative probabilities and the sum of probabilities over all possible values should be equal to one.
4. Some common discrete probability distributions include Bernoulli, Binomial, Geometric, and Poisson distributions.
5. Mean and variance are important measures of central tendency and variability of a probability distribution and can be calculated from the PMF.
6. Discrete random variables and probability mass functions have applications in various fields, including finance, healthcare, and engineering.
7. It's important to carefully evaluate the quality and accuracy of the data being used when working with discrete random variables and probability mass functions.

## Quiz

1. What is the probability mass function of a discrete random variable?

A. A function that assigns probabilities to intervals of values.

B. A function that assigns probabilities to individual values.

C. A function that measures the distance between values.

D. A function that measures the spread of values.

2. Which of the following is a discrete probability distribution?

A. Normal distribution

B. Uniform distribution

C. Exponential distribution

D. Poisson distribution

3. What is the mean of a discrete probability distribution?

A. The most common value in the distribution.

B. The midpoint of the distribution.

C. The average of all the values in the distribution.

D. The highest value in the distribution.

4. In which fields can discrete random variables and probability mass functions be used?

A. Finance, healthcare, and engineering.

B. Art, literature, and music.

C. Geography, history, and politics.

D. Sports, entertainment, and fashion.

Module 4: Random VariablesDiscrete Random Variable and Probability Mass Function

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