# The Binomial Distribution

## The Binomial Distribution Revision

**The Binomial Distribution**

The **Binomial distribution** is a distribution that tells us the **probability of a certain number of successes** given a **fixed success probability** in repeated trials. To understand what it is and how it works, we first need to understand **factorials** and **binomial coefficients**.

**Factorial**

The **factorial** of a number, written n! is equal to n\times (n-1)\times (n-2)\times ... \times 3\times 2\times 1

n! is **the number of ways you can arrange** \bm{n} **objects**.

You can picture this as having n choices for the first object, n-1 choices for the second object, n-2 choices for the third object, and so on.

**What if Some Objects are the Same?**

Suppose we are arranging n objects, and r of them are **the same**. In any given arrangement, we can swap two objects that are the same and **end up with the same arrangement**. Indeed, if r objects are the same, we can swap them into any order we like within the arrangement and we will not have changed the arrangement. There are r! such ways to swap r objects. So the number of arrangements of n objects, r of which are the same, is \dfrac{n!}{r!}

**Only Two Different Objects**

Suppose we want to arrange n objects, r of which are of one type and the remaining n-r are of a second type. We can arrange the r identical objects in r! ways and the n-r identical objects in (n-r)! ways. So overall we have \dfrac{n!}{r!(n-r)!} arrangements. This is known as the **binomial coefficient** of n and r, written as

**The Binomial Distribution**

The **Binomial distribution** tells us the probability of a x successes from n **independent events** where the probability of success for each event is p.

Its **probability function** is:

f(x)=\begin{pmatrix}n\\x\end{pmatrix}p^{x}(1-p)^{n-x}

We can understand this as \begin{pmatrix}n\\x\end{pmatrix} is the number of possible ways to have x successes out of n events, then we multiply by the probability of x successes and n-x failures, which is p^{x}(1-p)^{n-x}

**Notation: **If X is a **binomial random variable** with n trials and a success probability of p, we write X\sim B(n,p)

**Binomial Tables**

Your formula booklet may contain **binomial tables**. These give the **cumulative distribution function** value for the binomial distribution.

For example, if we want to find the probability of two or less successes out of five trials with a success probability of 0.15:

This shows that \mathbb{P}(X\leq 2)=0.9734

If your formula booklet does not have **binomial tables**, you are expected to **use the statistics functions of your calculator** to answer questions like this. **Make sure you are familiar with how these work, as they will be necessary for an exam.**

**When to use the Binomial Distribution**

A random variable X is **binomially distributed** if:

- There is a
**fixed number**of trials. - Each trial has
**only two outcomes**– success or failure. - The trials are
**independent**of each other. - The success
**probability is the same**in each trial. - X is the
**total number of successes**in all trials.

## The Binomial Distribution Example Questions

**Question 1: **Find \begin{pmatrix}6\\3\end{pmatrix}

**[2 marks]**

\begin{pmatrix}6\\3\end{pmatrix}=

\dfrac{6!}{3!\times 3!}=

\dfrac{6\times 5\times 4\times 3\times 2\times 1}{3\times 2\times 1\times 3\times 2\times 1}=

\dfrac{6\times 5\times 4\times \cancel{3}\times \cancel{2}\times \cancel{1}}{\cancel{3}\times \cancel{2}\times \cancel{1}\times 3\times 2\times 1}=

\dfrac{6\times 5\times 4}{3\times 2\times 1}=

\dfrac{6\times 5\times 4}{6}=

\dfrac{\cancel{6}\times 5\times 4}{\cancel{6}}=

5\times 4=

20

**Question 2: **What is \mathbb{P}(X=2) if X\sim B(5,0.25)?

**[2 marks]**

Use the formula:

\mathbb{P}(X=x)=\begin{pmatrix}n\\x\end{pmatrix}p^{x}(1-p)^{n-x}

Here, n=5,p=0.25,x=2

\begin{aligned}\mathbb{P}(X=2)&=\begin{pmatrix}5\\2\end{pmatrix}0.25^{2}(1-0.25)^{5-2}\\[1.2em]&=0.264\end{aligned}

**Question 3: **For X\sim B(12,0.4), use a binomial table or a calculator to find \mathbb{P}(X\leq 5)

**[1 mark]**

**Question 4: **Use a binomial table or a calculator to find \mathbb{P}(3\leq X\leq 6) where X\sim B(8,0.6)

**[1 mark]**

\mathbb{P}(3\leq X\leq 6)=

\mathbb{P}(X\leq 6)-\mathbb{P}(X\leq 2)

We can use the tables to find these values.

0.8936 - 0.0498 = 0.8438

**Question 5: **Every time Jeff turns on his television, it goes to a random channel. It can receive 40 channels. Jeff turns on his television 60 times in a month on average. What is the probability that it will show the channel Jeff wants to watch upon turning on at least three times in a month.

**[3 marks]**

Probability of correct channel is \dfrac{1}{40}, so we have X\sim B(60,\dfrac{1}{40}).

Want to find \mathbb{P}(X\geq 3)

\begin{aligned}\mathbb{P}(X\geq 3)&=1-\mathbb{P}(X\leq 2)\\[1.2em]&=1-0.8105\\[1.2em]&=0.1895\end{aligned}

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