Central Limit Theorem and Normal Distribution

Area Under Normal Distribution Curve
Area Under Normal Distribution Curve


Why is normal distribution is important?
To understand the question you have to go through the Central Limit Theorem.

Central Limit Theorem

According to central limit theorem if X1, X2, X3,……Xn are random variables drawn from any probability distribution function with mean  Σμi  and standard deviation Σσi where (i=1,2,3,……n). The sum of random variables X i.e X=X1+ X2 + X3+……+Xn with mean μ=Σμi and standard deviation σ=Σσi will approach to normal distribution.

Due to this theorem, this continuous probability distribution function is very popular  and has several applications in variety of fields.

Normal Distribution

A random variable X is said to follow normal distribution with two parameters μ and σ and is denoted by X~N(μ, σ²).  The normal distribution is also known as Gaussian distribution.

If it follows the following distribution function .

Further, a normal distribution   with normal variate Z is called standard normal distribution with mean μ=0 and standard deviation σ=1  i.e Z~N(0,1).

Z= (X-μ)/ σ


Normal Distribution Formula

                                                                            Normal Distribution Formula

Central Limit Theorem and Normal Distribution

Normal Distribution Curve

 Properties of Normal Distribution

  • Normal distribution curve is a bell shaped.
  • Normal distribution curve is symmetrical  in which mean=median=mode and coincides at center.
  • Skewness of normal distribution curve is 0.
  • The total area under normal distribution curve is 1.

Solved  Numerical Problems Related to Normal Distribution

Q-  If  X is normally distributed with mean 2 and standard deviation 9 then the calculate the probability distribution

P( 2<=X<=3) .


In question



σ²=1 i.e σ=1

Calculate Z using  formula Z= (X-μ)/ σ  for X=2

The we have Z=(2-2)/1= 0

Calculate Z using  formula Z= (X-μ)/ σ  for X=3

We get  Z=(3-2)/1=1

Then we get probability distribution  P(0<=Z<=1) corresponding to P( 2<=X<=3).


P(0<=Z<=1) = P(Z<=1) – P(Z<=0)

The table Z (Click on the link to see Z table https://www.ztable.net/ ) to calculate are under  P(Z<=1) – P(Z<=0)

=(0.50+0.3413)-(0.5+0.0)= 0.3413

You can see area covered by  P(0<=Z<=1) is  0.3413

Python Code to Plot Area Under Normal Distribution Curve

import numpy as np
import matplotlib.pyplot as plt
from scipy.stats import norm
x = np.arange(-4,4,0.001)
y = norm.pdf(x, mu, sigma)
z = x[(0 < x) & (x < 1.0)]
plt.plot(x, y)
plt.fill_between(z, 0, norm.pdf(z, mu, sigma))
plt.savefig(“Normal Distribution.png”)

The output of the program would be the image

Area Under Normal Distribution Curve

                                                          Area Under Normal Distribution Curve

Applications of Normal Distribution in Data Science and Machine Learning

  • SVM (Support Vector Machine) uses Gaussian kernel which is  based on normal distribution.
  • Gaussian Naive Bays classifier uses normal or Gaussian  distribution.
  • For hypothetical testing in statistics.


In this post, I have explained about normal distribution or Gaussian distribution which is a very famous continuous probability distribution function. I has lot applications in machine learning and data science. Hope you will understand and apply it.







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