Mathematics | Beta Distribution Model
Last Updated :
03 Sep, 2024
The Beta Distribution is a continuous probability distribution defined on the interval [0, 1], widely used in statistics and various fields for modeling random variables that represent proportions or probabilities. It is particularly useful when dealing with scenarios where the outcomes are bounded within a specific range, such as success rates, probabilities, and proportions in Bayesian inference, decision theory, and reliability analysis. The flexibility of the Beta Distribution allows it to take on various shapes depending on its parameters, making it a versatile tool in mathematical modeling.
The beta distribution is used to model continuous random variables whose range is between 0 and 1. For example, in Bayesian analyses, the beta distribution is often used as a prior distribution of the parameter p (which is bounded between 0 and 1) of the binomial distribution (see, e.g., Novick and Jackson, 1974).
Definition of the Beta Distribution
The Beta Distribution is parameterized by two positive shape parameters, α, and β, which determine the shape of the distribution.
The probability density function (PDF) of the Beta Distribution for a random variable X on the interval [0, 1] is given by:
f(x;α,β)= xα−1 (1−x)β−1 / B(α,β)
where 𝐵(𝛼,𝛽) is the Beta function, defined as:
B(α,β)=∫01 tα−1 (1−t)β−1 dt
This function serves as a normalization constant ensuring that the total probability over the interval [0, 1] equals 1.
Properties of the Beta Distribution
The Beta Distribution exhibits several key properties, which are essential for understanding its behavior and applications.
Probability Density Function (PDF)
The Probability Density Function (PDF) of the Beta Distribution describes the likelihood of a random variable X taking on a specific value within the interval [0, 1].
The shape of the PDF is influenced by the parameters α and 𝛽. Depending on these parameters, the distribution can be uniform, U-shaped, J-shaped, or bell-shaped, making it adaptable to various situations.
Probability Density FunctionCumulative Distribution Function (CDF)
The Cumulative Distribution Function (CDF) of the Beta Distribution, denoted as F(x;α,β), represents the probability that the random variable X is less than or equal to a specific value 𝑥.
It is computed as the integral of the PDF from 0 to 𝑥:
F(x;α,β)=∫0x f(t;α,β)dt
The CDF provides a way to calculate the probability of observing a value within a certain range, making it useful in hypothesis testing and confidence interval estimation.
Moments: Mean, Variance, Skewness, and Kurtosis
The moments of the Beta Distribution, including the mean, variance, skewness, and kurtosis, provide essential insights into the distribution's characteristics.
Mean:
The mean 𝐸[𝑋] of the Beta Distribution, which represents the central location or average value, is calculated as:
𝐸[𝑋]=𝛼/𝛼+𝛽
This formula shows that the mean is influenced by the relative values of the shape parameters 𝛼 and 𝛽. When 𝛼 equals 𝛽, the mean is 0.5, indicating a symmetric distribution centered around 0.5.
Variance:
The variance
Var(𝑋) measures the dispersion or spread of the distribution around the mean:
Var(𝑋)=𝛼𝛽 / (𝛼+𝛽)2(𝛼+𝛽+1)
A smaller variance indicates that the distribution is more concentrated around the mean, while a larger variance suggests a wider spread.
Skewness:
Skewness quantifies the asymmetry of the distribution. For the Beta Distribution, skewness is given by:
Skewness (𝑋)=2(𝛽−𝛼)√𝛼+𝛽+1/(𝛼+𝛽+2)√𝛼𝛽
- If 𝛼=𝛽, the distribution is symmetric (skewness = 0).
- If 𝛼>𝛽, the distribution is skewed to the left (negative skewness).
- If 𝛽>𝛼, the distribution is skewed to the right (positive skewness).
Kurtosis:
Kurtosis measures the "tailedness" or the concentration of data in the tails of the distribution:
Kurtosis(𝑋)=6[(𝛼−𝛽)2(𝛼+𝛽+1)𝛼𝛽(𝛼+𝛽+2)] / 𝛼𝛽(𝛼+𝛽+2)(𝛼+𝛽+3)
The kurtosis of the Beta Distribution can vary significantly based on the parameters 𝛼 and 𝛽, influencing the peak and tail behavior of the distribution.
Suppose an event can occur several times within a given unit of time. When the total number of occurrences of the event is unknown, we can think of it as a random variable. When a random variable X takes on values on the interval from 0 to 1, one choice of a probability density is the beta distribution whose probability density function is given by as follows.
Representation of probability density function -
f(x) = \frac{1}{\Beta(m.n)}x^{m-1}(1-x)^{n-1}
It will be applicable only when the given below given condition will pass.
x > 0, m >0, n >0.
f(x) = 0 , Otherwise
Here, you will see the meaning of function as you have shown in representation of probability density function where, B(m.n) is the value of the beta function.
Representation of B(m.n) -
\Beta(m,n) = \int^{1}_{0} x^{m- 1}(1-x)^{n-1}dx = \Beta(n,m)
Integrating it by parts, we will get the following expression as given below.
\Beta(m,n) = \frac{\Gamma(m)\Gamma(n)}{\Gamma(m+n)}
Where, Γ(x) is the gamma function of x, calculated as -
Γ(x) = (x-1)Γ(x-1) for x>1
=> \Gamma(x) = (x-1)! when x is an integer
The random variable X is represented as follows.
Representation of random variable X -
X ~ BETA(m,n)
Expected Value:
The Expected Value of the Beta distribution can be found by summing up products of Values with their respective probabilities.
\mu = E(X) = \int^{\infty}_{-\infty} x.f(x) dx
\mu = \frac{1}{\Beta(m,n)} \int^{1}_{0}x. x^{m - 1}(1-x)^{n-1} dx
\mu = \frac{1}{\Beta(m,n)} \int^{1}_{0} x^{m}(1-x)^{n-1} dx
Upon using the value of beta function, we will get the following expression a follows.
\mu = \frac{1}{\Beta(m,n)} \Beta(m+1,n)
but, \Beta(m,n) = \frac{\Gamma(m)\Gamma(n)}{\Gamma(m+n)}
also, \Beta(m+1,n) = \frac{\Gamma(m+1)\Gamma(n)}{\Gamma(m+n+1)}
=> \mu = \frac{\Gamma(m+1)\Gamma(n)}{\Gamma(m+n+1)} * \frac{\Gamma(m+n)}{\Gamma(m)\Gamma(n)}
Upon using the property of gamma function, that is Γ(x) = (x-1)!, we will get the following expression as follows.
So, E(x) = \frac{m! (n-1)! (m+n-1)!}{(m-1)! (n-1)! (m+n)!}
here, m, n -> integers
where provided m and n are integers.
Variance and Standard Deviation
The Variance of the Beta distribution can be found using the Variance Formula.
σ^2 = E( X − μ )^2 = E( X^2 ) − μ^2
E(X^2) = \int^{\infty}_{-\infty} x^2.f(x) dx
E(X^2) = \int^{1}_{0}\frac{1}{\Beta(m,n)}x^2. x^{m - 1}(1-x)^{n-1} dx
E(X^2) = \frac{1}{\Beta(m,n)}\int^{1}_{0}x^{m + 1}(1-x)^{n-1} dx
Upon using the value of beta function, we will get the following expression as follows.
E(X^2) = \frac{1}{\Beta(m,n)} \Beta(m+2,n+2)
but, \Beta(m,n) = \frac{\Gamma(m)\Gamma(n)}{\Gamma(m+n)}
also, \Beta(m+1,n) = \frac{\Gamma(m+2)\Gamma(n)}{\Gamma(m+n+2)}
=> \mu = \frac{\Gamma(m+2)\Gamma(n)}{\Gamma(m+n+2)} * \frac{\Gamma(m+n)}{\Gamma(m)\Gamma(n)}
Upon using the property of gamma function, that is Γ(x) = (x-1)!, we will get the following expression as follows.
E(X^2) = \frac{(m+1)!(n-1)!(m+n-1)!}{(m+n+1)!(m-1)!(n-1)!}
E(X^2) = \frac{m(m+1)}{(m+n+1)(m+n)}
So, Var(X) = E(X^2) - \mu^2
Var(X) = \frac{m(m+1)}{(m+n+1)(m+n)} - \frac{m^2}{(m+n)^2}
Var(X) = \sigma^2= \frac{m.n}{(m+n+1)(m+n)^2}
Standard Deviation is given by as follows.
\sigma = \sqrt{\frac{m.n}{(m+n+1)(m+n)^2}} = \frac{1}{(m+ n)}\sqrt{\frac{m.n}{(m+n+1)}}
Example -
In a certain county, the proportion of highway sections requiring repairs in any given year is a random variable having the beta distribution with m = 3 and n = 2.
(a) On the average, what percentage of the highway sections require repairs in any given year?
(b) Find the probability that at most half of the highway sections will require repairs in any given year.
Solution -
(a) \mu = \frac{3}{3+2} = 0.60
which means that on the average 60% of the highway sections require repairs in any given year.
(b) Substituting m=3 and n=2 in the probability density function, and
using Γ(3) = 2! = 2, Γ(2) = 1! = 1, Γ(5) = 4! = 24,
we will get the following expression as follows.
f(x) = 12 x^2 (1 − x) for 0<x<1. Otherwise f(x) = 0
Thus, the desired probability is given by as follows.
\int^{\frac{1}{2}}_0 12x^2 (1-x) dx = \frac{5}{16}
Shape of the Beta Distribution
The shape of the Beta Distribution is determined by the parameters
𝛼 and 𝛽, allowing it to take on various forms:
Uniform Distribution:
When 𝛼=𝛽=1, the Beta Distribution becomes a uniform distribution on the interval [0, 1], meaning every value within this interval is equally likely.
Bell-Shaped Distribution:
When 𝛼>1 and 𝛽>1, the Beta Distribution takes on a bell-shaped, unimodal form. The peak of the distribution occurs at 𝛼−1 / 𝛼+𝛽−2, making it suitable for modeling distributions with a central tendency.
U-Shaped Distribution:
When 𝛼<1 and 𝛽<1, the distribution is U-shaped, with most of the probability mass concentrated at the boundaries 0 and 1. This form is useful for modeling extreme probabilities.
J-Shaped Distribution:
When 𝛼<1 and 𝛽>1, the distribution is skewed towards 0, resembling a J-shape.
When 𝛼>1 and 𝛽<1, the distribution is skewed towards 1, also taking on a J-shaped form.
Applications of the Beta Distribution
The Beta Distribution's versatility makes it valuable across various fields:
Bayesian Inference:
The Beta Distribution is frequently used as a prior distribution in Bayesian analysis, especially for modeling binary outcomes and probabilities. Its conjugate nature with the binomial distribution simplifies posterior calculations.
Project Management (PERT):
In project management, the Beta Distribution is used in the Program Evaluation and Review Technique (PERT) to model the distribution of task completion times. It helps in estimating the expected duration and the likelihood of meeting deadlines.
Reliability Engineering:
The Beta Distribution is applied to model the life expectancy and failure rates of systems and components, particularly when data is sparse or when modeling expert opinions.
Genetics and Population Studies:
The Beta Distribution is employed to model the distribution of allele frequencies in populations, providing insights into genetic diversity and evolutionary dynamics.
Quality Control:
In quality control processes, the Beta Distribution is used to model the distribution of defect rates, helping organizations understand and manage variability in production processes.
The Beta Distribution's ability to model proportions, probabilities, and bounded outcomes makes it a powerful tool in both theoretical and applied statistics.
Solved Examples on Beta Distribution Model
Example 1: For a Beta Distribution with parameters α = 2 and β = 5:
- Mean: Calculate using the formula
α / (α + β). Thus, the mean is 2 / (2 + 5) = 2 / 7 ≈ 0.286. - Variance: Use the formula
αβ / [(α + β)² (α + β + 1)]. Therefore, the variance is 2 * 5 / [7² * 8] = 10 / 49 * 8 ≈ 0.0255.
Example 2: In a survey with α = 3 and β = 7, find the probability that the proportion is less than 0.4:
- Solution: To find this, use the cumulative distribution function (CDF) of the Beta Distribution evaluated at 0.4 with the given parameters.
Example 3: For α = 4 and β = 4, compute the skewness:
- Skewness: Using the formula
2 (β - α) sqrt(α + β + 1) / [(α + β + 2) sqrt(αβ)], the skewness is 2 * (4 - 4) * sqrt(8) / [(10) * sqrt(16)] = 0 because α equals β.
Example 4: With α = 1 and β = 2, find the probability that X is between 0.2 and 0.5:
- Solution: Calculate the difference between the CDF values at 0.5 and 0.2 using the given parameters.
Example 5: For α = 5 and β = 3, compute the kurtosis:
- Kurtosis: Substitute α and β into the kurtosis formula and simplify to obtain the value.
Practice Problems
- Compute the mean and variance for a Beta Distribution with α = 7 and β = 3.
- Find the probability that X is less than 0.6 for a Beta Distribution with α = 4 and β = 5.
- Determine the skewness of a Beta Distribution with α = 2 and β = 6.
- For α = 6 and β = 2, calculate the probability that X is greater than 0.3.
- Find the kurtosis for a Beta Distribution with α = 3 and β = 8.
- Compute the mean for a Beta Distribution where α = 10 and β = 15.
- Calculate the variance of a Beta Distribution with α = 5 and β = 5.
- Determine the CDF value at x = 0.7 for α = 2 and β = 3.
- Find the probability that X is between 0.4 and 0.8 for α = 4 and β = 6.
- Compute the skewness of a Beta Distribution with α = 8 and β = 4.
Summary / Conclusion
The Beta Distribution is a powerful and flexible tool used in statistics to model random variables constrained within the interval [0, 1]. Its versatility, governed by the parameters α and β, allows it to model various shapes of distributions, from uniform to bell-shaped, U-shaped, and J-shaped. The distribution's mean, variance, skewness, and kurtosis provide deep insights into its behavior, making it suitable for diverse applications such as Bayesian inference, project management, reliability engineering, genetics, and quality control. Understanding these properties helps in effective modeling and analysis of bounded outcomes.
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Probability distribution is a mathematical function that can be thought of as providing the probabilities of occurrence of different possible outcomes in an experiment. For instance, if the random variable X is used to denote the outcome of a coin toss ("the experiment"), then the probability distri
3 min read
Mathematics | Probability Distributions Set 4 (Binomial Distribution)
The previous articles talked about some of the Continuous Probability Distributions. This article covers one of the distributions which are not continuous but discrete, namely the Binomial Distribution.Introduction -To understand the Binomial distribution, we must first understand what a Bernoulli T
5 min read
Mathematics | Probability Distributions Set 5 (Poisson Distribution)
The previous article covered the Binomial Distribution. This article talks about another Discrete Probability Distribution, the Poisson Distribution.Introduction -Suppose an event can occur several times within a given unit of time. When the total number of occurrences of the event is unknown, we ca
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Uniform Distribution | Formula, Definition and Examples
A Uniform Distribution is a type of probability distribution in which every outcome in a given range is equally likely to occur. That means there is no bias—no outcome is more likely than another within the specified set.It is also known as rectangular distribution (continuous uniform distribution).
11 min read
Mathematics | Probability Distributions Set 2 (Exponential Distribution)
The previous article covered the basics of Probability Distributions and talked about the Uniform Probability Distribution. This article covers the Exponential Probability Distribution which is also a Continuous distribution just like Uniform Distribution. Introduction - Suppose we are posed with th
5 min read
Mathematics | Probability Distributions Set 3 (Normal Distribution)
The previous two articles introduced two Continuous Distributions: Uniform and Exponential. This article covers the Normal Probability Distribution, also a Continuous distribution, which is by far the most widely used model for continuous measurement.Introduction -Whenever a random experiment is rep
5 min read
Mathematics | Beta Distribution Model
The Beta Distribution is a continuous probability distribution defined on the interval [0, 1], widely used in statistics and various fields for modeling random variables that represent proportions or probabilities. It is particularly useful when dealing with scenarios where the outcomes are bounded
11 min read
Gamma Distribution Model in Mathematics
Introduction : Suppose an event can occur several times within a given unit of time. When the total number of occurrences of the event is unknown, we can think of it as a random variable. Now, if this random variable X has gamma distribution, then its probability density function is given as follows
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Chi-Square Test for Feature Selection - Mathematical Explanation
One of the primary tasks involved in any supervised Machine Learning venture is to select the best features from the given dataset to obtain the best results. One way to select these features is the Chi-Square Test. Mathematically, a Chi-Square test is done on two distributions two determine the lev
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Student's t-distribution in Statistics
As we know normal distribution assumes two important characteristics about the dataset: a large sample size and knowledge of the population standard deviation. However, if we do not meet these two criteria, and we have a small sample size or an unknown population standard deviation, then we use the
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Python - Central Limit Theorem
Central Limit Theorem (CLT) is a foundational principle in statistics, and implementing it using Python can significantly enhance data analysis capabilities. Statistics is an important part of data science projects. We use statistical tools whenever we want to make any inference about the population
7 min read
Limits, Continuity and Differentiability
Limits, Continuity, and Differentiation are fundamental concepts in calculus. They are essential for analyzing and understanding function behavior and are crucial for solving real-world problems in physics, engineering, and economics.Table of ContentLimitsKey Characteristics of LimitsExample of Limi
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Implicit Differentiation
Implicit Differentiation is the process of differentiation in which we differentiate the implicit function without converting it into an explicit function. For example, we need to find the slope of a circle with an origin at 0 and a radius r. Its equation is given as x2 + y2 = r2. Now, to find the s
5 min read
Calculus for Machine Learning
Partial Derivatives in Engineering Mathematics
Partial derivatives are a basic concept in multivariable calculus. They convey how a function would change when one of its input variables changes, while keeping all the others constant. This turns out to be particularly useful in fields such as physics, engineering, economics, and computer science,
10 min read
Advanced Differentiation
Derivatives are used to measure the rate of change of any quantity. This process is called differentiation. It can be considered as a building block of the theory of calculus. Geometrically speaking, the derivative of any function at a particular point gives the slope of the tangent at that point of
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How to find Gradient of a Function using Python?
The gradient of a function simply means the rate of change of a function. We will use numdifftools to find Gradient of a function. Examples: Input : x^4+x+1 Output :Gradient of x^4+x+1 at x=1 is 4.99 Input :(1-x)^2+(y-x^2)^2 Output :Gradient of (1-x^2)+(y-x^2)^2 at (1, 2) is [-4. 2.] Approach: For S
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Optimization techniques for Gradient Descent
Gradient Descent is a widely used optimization algorithm for machine learning models. However, there are several optimization techniques that can be used to improve the performance of Gradient Descent. Here are some of the most popular optimization techniques for Gradient Descent: Learning Rate Sche
4 min read
Higher Order Derivatives
Higher order derivatives refer to the derivatives of a function that are obtained by repeatedly differentiating the original function.The first derivative of a function, f′(x), represents the rate of change or slope of the function at a point.The second derivative, f′′(x), is the derivative of the f
6 min read
Taylor Series
A Taylor series represents a function as an infinite sum of terms, calculated from the values of its derivatives at a single point.Taylor series is a powerful mathematical tool used to approximate complex functions with an infinite sum of terms derived from the function's derivatives at a single poi
8 min read
Application of Derivative - Maxima and Minima
Derivatives have many applications, like finding rate of change, approximation, maxima/minima and tangent. In this section, we focus on their use in finding maxima and minima.Note: If f(x) is a continuous function, then for every continuous function on a closed interval has a maximum and a minimum v
6 min read
Absolute Minima and Maxima
Absolute Maxima and Minima are the maximum and minimum values of the function defined on a fixed interval. A function in general can have high values or low values as we move along the function. The maximum value of the function in any interval is called the maxima and the minimum value of the funct
11 min read
Optimization for Data Science
From a mathematical foundation viewpoint, it can be said that the three pillars for data science that we need to understand quite well are Linear Algebra, Statistics and the third pillar is Optimization which is used pretty much in all data science algorithms. And to understand the optimization conc
5 min read
Unconstrained Multivariate Optimization
Wikipedia defines optimization as a problem where you maximize or minimize a real function by systematically choosing input values from an allowed set and computing the value of the function. That means when we talk about optimization we are always interested in finding the best solution. So, let sa
4 min read
Lagrange Multipliers | Definition and Examples
In mathematics, a Lagrange multiplier is a potent tool for optimization problems and is applied especially in the cases of constraints. Named after the Italian-French mathematician Joseph-Louis Lagrange, the method provides a strategy to find maximum or minimum values of a function along one or more
8 min read
Lagrange's Interpolation
What is Interpolation? Interpolation is a method of finding new data points within the range of a discrete set of known data points (Source Wiki). In other words interpolation is the technique to estimate the value of a mathematical function, for any intermediate value of the independent variable. F
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Linear Regression in Machine learning
Linear regression is a type of supervised machine-learning algorithm that learns from the labelled datasets and maps the data points with most optimized linear functions which can be used for prediction on new datasets. It assumes that there is a linear relationship between the input and output, mea
15+ min read
Ordinary Least Squares (OLS) using statsmodels
Ordinary Least Squares (OLS) is a widely used statistical method for estimating the parameters of a linear regression model. It minimizes the sum of squared residuals between observed and predicted values. In this article we will learn how to implement Ordinary Least Squares (OLS) regression using P
3 min read
Regression in Machine Learning