# 4.4 The binomial random variable

 Page 1 / 1

When you flip a coin, there are two possible outcomes: heads and tails. Each outcome has a fixed probability, the same from trialto trial. In the case of coins, heads and tails each have the same probability of 1/2. More generally, there are situations inwhich the coin is biased, so that heads and tails have different probabilities. In the present section, we consider probabilitydistributions for which there are just two possible outcomes with fixed probability summing to one. These distributions arecalled are called binomial distributions .

## A simple example

The four possible outcomes that could occur if you flipped a coin twice are listed in [link] . Note that the four outcomes are equally likely: each has probability $1/4$ . To see this, note that the tosses of the coin are independent(neither affects the other). Hence, the probability of a head on Flip 1 and a head on Flip 2 is the product of $\mathrm{P\left[H\right]}()$ and $\mathrm{P\left[H\right]}()$ , which is $1/2\times 1/2=1/4$ . The same calculation applies to the probability of a head on Flip one and a tail on Flip 2. Each is $1/2\times 1/2=1/4$ .

Four possible outcomes
Outcome First Flip Second Flip
4 Tails Tails

The four possible outcomes can be classifid in terms of the number of heads that come up. The number could betwo (Outcome 1), one (Outcomes 2 and 3) or 0 (Outcome 4). The probabilities of these possibilities are shown in [link] and in [link] . Since two of the outcomes represent the case in which just one head appears inthe two tosses, the probability of this event is equal to $1/4+1/4=1/2$ . [link] summarizes the situation.

Probabilities of getting 0,1, or 2 heads.
0 1/4
1 1/2
2 1/4

[link] is a discrete probability distribution: It shows the probability for each of the values on theX-axis. Defining a head as a "success," [link] shows the probability of 0, 1, and 2 successes for two trials (flips) for an event that has a probability of 0.5 of being asuccess on each trial. This makes [link] an example of a binomial distribution .

## The formula for binomial probabilities

The binomial distribution consists of the probabilities of each of the possible numbers of successes on $n$ trials for independent events that each have a probability of $p()$ of occurring. For the coin flip example, $n=2$ and $\mathrm{p=}(0.5)$ . The formula for the binomial distribution is shown below: $\mathrm{P\left[x\right]}()=\frac{n!}{x!(n-x)!}p^{x}(1-p)^{(n-x)}$ where $\mathrm{P\left[x\right]}()$ is the probability of $x$ successes out of $n$ trials, $n$ is the number of trials, and $p$ is the probability of success on a given trial. Applying this to thecoin flip example, $P\left[0\right]()=\frac{2!}{0!(2-0)!}0.5^{0}(1-0.5)^{(2-0)}=\frac{2}{2}\times 1\times .25=0.25$ $P\left[1\right]()=\frac{2!}{1!(2-1)!}0.5^{1}(1-0.5)^{(2-1)}=\frac{2}{1}\times .5\times .5=0.50$ $P\left[2\right]()=\frac{2!}{2!(2-2)!}0.5^{2}(1-0.5)^{(2-2)}=\frac{2}{2}\times .25\times 1=0.25$ If you flip a coin twice, what is the probability of getting one or more heads? Since the probability of getting exactlyone head is 0.50 and the probability of getting exactly two heads is 0.25, the probability of getting one or more heads is $0.50+0.25=0.75$ .

Now suppose that the coin is biased; let's say the probability of heads is only 0.4. What is the probability of getting heads at leastonce in two tosses? We could substitute p=0.4 with x=1 and with x=2 into our general formula above; adding the results would obtain the answer 0.64.

## Cumulative probabilities

We toss a coin 12 times. What is the probability that we get from 0 to 3 heads? The answer is found by computing theprobability of exactly 0 heads, exactly 1 head, exactly 2 heads, and exactly 3 heads. The probability of getting from 0to 3 heads is then the sum of these probabilities. The probabilities are: 0.0002, 0.0029, 0.0161, and 0.0537. The sumof the probabilities is 0.073. The calculation of cumulative binomial probabilities can be quite tedious. Therefore we haveprovided a binomial calculator to make it easy to calculate these probabilities.

## Mean and standard deviation of binomial distributions

Consider a coin-tossing experiment in which you tossed a coin 12 times and recorded the number of heads. If you performedthis experiment over and over again, what would the mean number of heads be? On average, you would expect half the cointosses to come up heads. Therefore the mean number of heads would be 6. In general, the mean of a binomial distributionwith parameters $n$ (the number of trials) and $p$ (the probability of success for each trial) is: $\mu =np$ where $\mu$ is the mean of the binomial distribution. The variance of the binomial distribution is: $\sigma ^{2}=np(1-p)$ where $\sigma ^{2}$ is the variance of the binomial distribution.

Let's return to the coin tossing experiment. The coin was tossed 12 times so $n=12$ . A coin has a probability of 0.5 of coming up heads. Therefore, $p=0.5$ . The mean and standard deviation can therefore be computed as follows: $\mu =np=12\times 0.5=6$ $\sigma ^{2}=np(1-p)=12\times 0.5(1.0-0.5)=3.0$ Naturally, the standard deviation $\left(\sigma \right)$ is the square root of the variance $\left(\sigma ^{2}\right)$ .

where we get a research paper on Nano chemistry....?
what are the products of Nano chemistry?
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
learn
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
learn
da
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Bhagvanji
Preparation and Applications of Nanomaterial for Drug Delivery
revolt
da
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
I only see partial conversation and what's the question here!
what about nanotechnology for water purification
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
Damian
yes that's correct
Professor
I think
Professor
Nasa has use it in the 60's, copper as water purification in the moon travel.
Alexandre
nanocopper obvius
Alexandre
what is the stm
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
Rafiq
if virus is killing to make ARTIFICIAL DNA OF GRAPHENE FOR KILLED THE VIRUS .THIS IS OUR ASSUMPTION
Anam
analytical skills graphene is prepared to kill any type viruses .
Anam
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
Hafiz
what is Nano technology ?
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
why we need to study biomolecules, molecular biology in nanotechnology?
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
why?
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
how did you get the value of 2000N.What calculations are needed to arrive at it
Privacy Information Security Software Version 1.1a
Good
Got questions? Join the online conversation and get instant answers!