# Bayes Theorem of Probability

## Bayes’ Theorem Introduction

Bayes’ Theorem governs the likelihood that one event is based on the occurrence of some other events. It depends upon the concepts of conditional probability. This theorem gives us the probability of some events depending on some conditions related to the event.

We know that the likelihood of heart disease increases with increasing age. So, if we know someone’s age and know the chances of getting heart disease, we can easily find the opportunity of the person having a heart disease.

Let us take another example. We have been given a bag of green, white and red balls. It’s been said that we have to pick a green ball only after picking a blue ball. It’s been sure that we can only pick green balls after picking a blue ball. This makes the case of conditional probability.

## Formula of Bayes’ Theorem

P(A|B) = $\frac{P(A).P(B|A)}{P(B)}$

Where:

P(A) = Probability of occurrence of event A

P(B) = Probability of occurrence of event B

P(A|B) = Probability of occurrence of event A given B

P(B|A) = Probability of occurrence of event B given A

## Bayes’ Theorem Statement

Let B1, B2, B3, ….., Bn is a series of n non-empty sets with sample space as X, where all the sets have a non-zero probability of occurrence, and they form a partition of X. Let E be an event associated with X, then by Bayes’ Theorem we get:

$P(B_i|X) = \frac{P(B_i)P(X|B_i)}{\sum_{j=1}^n P(B_j)P(A|B_j)}$

For all i = 1, 2, 3, ……,n

### Bayes Theorem Derivation

According to conditional probability, we get,

$P(B_i|X) = \frac{P(B_i)P(X|B_i)}{P(A)}$ [By multiplicative property]

= $\frac{P(B_i)P(X|B_i)}{\sum_{j=1}^n P(B_j)P(X|B_j)}$ [By Total Probability Theorem]

Each of these occurrences B1, B2, B3, ….., Bn is called a hypothesis.

The probability P(Bi) is known as a prior probability of the hypothesis Bi.

The probability of P(Bi|X) is called a posteriori probability of the hypothesis Bi.

The significance of Baye’s theorem can be understood in the following manner.

The experiment can be performed in n mutually exclusive and exhaustive ways B1,B2…Bn. The probability P(Bi) of the occurrence of event Bi ; i = 1,2,..n is known. The experiment is done and we  know that event X has occurred. With this information the probability P(Bi) is changed to P(Bi/X). This theorem enables us to evaluate P(Bi/X) if all the P(Bi) priori probabilities and P(X/Bi) likelihood probabilities are known.

### Bayes Theorem Problems

Example 1: Consider 2 bags. Bag 1 is made full by putting three red and four blue balls. Bag 2 is made full by putting four red and six blue balls. We have to find out the value of the probability of drawing a red ball from the second bag. So, we have to find out from which bag the red ball will be drawn.

Solution: Let us take B1 to be chance of choosing Bag 1;

B2to be the probability of choosing Bag 2;

A be the probability of drawing a red ball;

We know the probability of selecting any bag is equal to [P(B1) = P(B2)] = 1/2;

Probability of drawing a red ball from the first bag = [P(A| B1)] = 3/7;

Probability of drawing a red ball from the second bag = [P(A| B2)] = 4/10;

So, the probability of drawing a red ball from the second bag will be P(B2|A).

Example 2:

Consider there are three machines. All of the machines can produce 1000 pins at a time. The rate of producing a faulty pin from Machine 1 be 10%, from Machine 2 be 20% and from Machine 3 be 5%. What is the probability that a produced pin will be faulty and it will be from the first machine?

Solution:

Let us take the probability of choosing a faulty pin randomly be represented by P(A);

Pin choose from the first machine be represented by M1;

Pin choose from the second machine be represented by M2:

Pin choose from the third machine be represented by M3:

Chance of choosing pin any one of the three machines = P(M1) = P(M2) = P(M3) = 1/3

Probability of choosing a faulty pin from 1st machine is

Example 3:

You have been given dice and a pack of 52 cards. You have to throw a dice and then you have to pick up a card. What is the probability that you picked up a red card and threw 6 on the dice?

Solution:

Let us denote the probability of throwing 6 on an unbiased dice be represented by P(D);

So, P(D) = 1/6

So, the probability of throwing another number on the dice is represented by P(E) = 5/6

The probability of picking up a red card=and it is denoted by P(R1) = 1/2

So, the probability of not picking up a red card = 1 – ½ = ½

and it is denoted by P(R2) = 1/2

So, the probability of picking up a red card after drawing a 6 on the dice

Example 4:

You have been given a dice. You tell a lie 4 out of 5 times. It is reported that an odd number on the dice appears. Find out the probability that really an odd number appears.

Solution:

Let the probability of speaking truth be represented by P(T) and its value is 4/5

So, the probability of speaking a lie is represented by P(L) and its value is 1 – 4/5 = 1/5

Let the probability of an odd number appearing on the dice is represented by P(0) and its value is given by 3/6 or 1/2 (as there are 3 odd numbers on the dice- 1,3,5)

Probability of an even number appearing is denoted by P(E) and its value is given by 1-1/2 = 1/2

Therefore, probability of an odd number really appearing will be given by

$P(O|T) = \frac{P(O)P(T|O)}{P(O)P(T|O) + P(E)P(T|E)}$

= $\frac{\frac{1}{2} \times \frac{4}{5}}{\frac{1}{2} \times \frac{4}{5} + \frac{1}{2} \times \frac{1}{5}}$

= 4/5

Example 5: If A and B are two events such that $P\,(A)\ne 0 \ and \ P\,(B)\ne 1, \text \ then \ P\,\left( \frac{{\bar{A}}}{{\bar{B}}} \right)=$

Solution:

$P\left( \frac{\overline{A}}{\overline{B}} \right)=\frac{P(\overline{A}\cap \overline{B})}{P(\bar{B})}=\frac{P(\overline{A\cup B})}{P(\bar{B})}=\frac{1-P(A\cup B)}{P(\bar{B})}\\$

Example 6: Find the  condition for E and F to be independent events such that $0 and $0.

Solution:

$P(E\cap F)=P(E)\,.\,P(F)$

Now, $P(E\cap {{F}^{c}})=P(E)-P(E\cap F)=P(E)[1-P(F)]=P(E)\,.P({{F}^{c}})$ and $P({{E}^{c}}\cap {{F}^{c}})=1-P(E\cup F)=1-[P(E)+P(F)-P(E\cap F) \\=[1-P(E)][1-P(F)]=P({{E}^{c}})\,P({{F}^{c}})$

Also $P(E/F)=P(E)$ and $P({{E}^{c}}/{{F}^{c}})=P({{E}^{c}}) \Rightarrow P(E/F)+P({{E}^{c}}/{{F}^{c}})=1$

Example 7: For a biased die, the probabilities for different faces to turn up are

 Face : 1 2 3 4 5 6 Probability : 0.2 0.22 0.11 0.25 0.05 0.17

The die is tossed and you are told that either face 4 or face 5 has turned up. What is the probability that it faces 4?

Solution:

Let A be the event that faces 4 turns up and B be the event that faces 5 turns up then P (A) = 0.25, P (B) = 0.05.

Since A and B are mutually exclusive, so P (A ∪ B) = P (A) + P (B) = 0.25 + 0.05 = 0.30.

We have to find $P\left( \frac{A}{A\cup B} \right),$ which is equal to $P\frac{[A\cap (A\cup B)]}{P(A\cup B)}=\frac{P(A)}{P(A\cup B)}=\frac{0.25}{0.30}=\frac{5}{6}$

Example 7: In a bolt factory, machines A, B and C manufactures respectively 25%, 35% and 40% of the total bolts. Of their outputs, 5,4 and 2 percent are respectively defective bolts. A bolt is drawn at random from the product. If the bolt drawn is found to be defective, what is the probability that it is manufactured by machine B.

Solution:

Let B1 be the event that the bolt is manufactured by machine A.

B2 be the event that the bolt is manufactured by machine B.

B3 be the event that the bolt is manufactured by machine C.

X be the event that the bolt is defective.

P(B1) = 25/100

P(B2) = 35/100

P(B3) = 40/100

P(X/B1) = Probability that the bolt drawn is defective so that it is manufactured by machine A = 5/100

P(X/B2) = Probability that the bolt drawn is defective so that it is manufactured by machine B = 4/100

P(X/B3) = Probability that the bolt drawn is defective so that it is manufactured by machine C = 2/100

Therefore the probability that the bolt is manufactured by machine B given that the bolt drawn is defective = P(B2/X)

= P(B2) P(X/B2) ÷ [P(B1)P(X/B1) +P(B2)P(X/B2)+P(B3)P(X/B3)]

= (35/100)(4/100) ÷ [(25/100)(5/100)+(35/100)(4/100)+(40/100)(2/100)]

= 140/(125+140+80) = 140/345 = 28/69

Hence the required probability is 28/69.

Example 8: Bag A contains 3 red and 4 black balls and bag B contains 4 red and 5 black balls. One ball is transferred from bag A to bag B and then a ball is drawn from bag B. The ball so drawn is found to be red in colour. Find the probability that the transferred ball is black.

Solution:

Let E1 be the event that the ball transferred from bag A to B is red,

E2 be the event that the ball transferred from bag A to B is black and A be the event that the ball drawn from bag B is in red colour.

P(E1) = 3/7

P(E2) = 4/7

P(A/E1) = 5/10 = ½

P(A/E2) = 4/10 = ⅖

So Probability that transferred ball is black = P(E2/A)

= P(E2) P(A/E2) ÷ [ P(E1) P(A/E1)+P(E2) P(A/E2)

= (4/7) ( 2/5) ÷ [(3/7)(½)+(4/7)(⅖) = 16/31

Hence the required probability is 16/31.