# Standard deviation and variance  (Page 3/3)

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 $\overline{X}$ Deviation $\left(X-\overline{X}\right)$ Deviation squared ${\left(X-\overline{X}\right)}^{2}$ 57 1 1 53 -3 9 58 2 4 65 9 81 48 -8 64 50 -6 36 66 10 100 51 -5 25 $\sum X=448$ $\sum x=0$ $\sum {\left(X-\overline{X}\right)}^{2}=320$

Note: The sum of the deviations of scores about their mean is zero. This always happens; that is $\left(X-\overline{X}\right)=0$ , for any set of data. Why is this? Find out.

Calculate the variance (add the squared results together and divide this total by the number of items).

$\begin{array}{ccc}\hfill \mathbf{Variance}& =& \frac{\sum {\left(X-\overline{X}\right)}^{2}}{n}\hfill \\ & =& \frac{320}{8}\hfill \\ & =& 40\hfill \end{array}$
$\begin{array}{ccc}\hfill \mathbf{Standard deviation}& =& \sqrt{\mathrm{variance}}\hfill \\ & =& \sqrt{\frac{\sum {\left(X-\overline{X}\right)}^{2}}{n}}\hfill \\ & =& \sqrt{\frac{320}{8}}\hfill \\ & =& \sqrt{40}\hfill \\ & =& 6.32\hfill \end{array}$

## Difference between population variance and sample variance

As with variance, there is a distinction between the standard deviation, $\sigma$ , of a whole population and the standard deviation, $s$ , of sample extracted from the population.

When dealing with the complete population the (population) standard deviation is a constant, a parameter which helps to describe the population. When dealing with a sample from the population the (sample) standard deviation varies from sample to sample.

In other words, the standard deviation can be calculated as follows:

1. Calculate the mean value $\overline{x}$ .
2. For each data value ${x}_{i}$ calculate the difference ${x}_{i}-\overline{x}$ between ${x}_{i}$ and the mean value $\overline{x}$ .
3. Calculate the squares of these differences.
4. Find the average of the squared differences. This quantity is the variance, ${\sigma }^{2}$ .
5. Take the square root of the variance to obtain the standard deviation, $\sigma$ .

What is the variance and standard deviation of the population of possibilities associated with rolling a fair die?

1. When rolling a fair die, the population consists of 6 possible outcomes. The data set is therefore $x=\left\{1,2,3,4,5,6\right\}$ . and n=6.

2. The population mean is calculated by:

$\begin{array}{ccc}\hfill \overline{x}& =& \frac{1}{6}\left(1+2+3+4+5+6\right)\hfill \\ & =& 3,5\hfill \end{array}$
3. The population variance is calculated by:

$\begin{array}{ccc}\hfill {\sigma }^{2}& =& \frac{\sum {\left(x-\overline{x}\right)}^{2}}{n}\hfill \\ & =& \frac{1}{6}\left(6,25+2,25+0,25+0,25+2,25+6,25\right)\hfill \\ & =& 2,917\hfill \end{array}$
4.  $\overline{X}$ $\left(X-\overline{X}\right)$ ${\left(X-\overline{X}\right)}^{2}$ 1 -2.5 6.25 2 -1.5 2.25 3 -0.5 0.25 4 0.5 0.25 5 1.5 2.25 6 2.5 6.25 $\sum X=21$ $\sum x=0$ $\sum {\left(X-\overline{X}\right)}^{2}=17.5$
5. The (population) standard deviation is calculated by:

$\begin{array}{ccc}\hfill \sigma & =& \sqrt{2,917}\hfill \\ & =& 1,708.\hfill \end{array}$

Notice how this standard deviation is somewhere in between the possible deviations.

## Interpretation and application

A large standard deviation indicates that the data values are far from the mean and a small standard deviation indicates that they are clustered closely around the mean.

For example, each of the three samples (0, 0, 14, 14), (0, 6, 8, 14), and (6, 6, 8, 8) has a mean of 7. Their standard deviations are 8.08, 5.77 and 1.15, respectively. The third set has a much smaller standard deviation than the other two because its values are all close to 7. The value of the standard deviation can be considered large' or small' only in relation to the sample that is being measured. In this case, a standard deviation of 7 may be considered large. Given a different sample, a standard deviation of 7 might be considered small.

Standard deviation may be thought of as a measure of uncertainty. In physical science for example, the reported standard deviation of a group of repeated measurements should give the precision of those measurements. When deciding whether measurements agree with a theoretical prediction, the standard deviation of those measurements is of crucial importance: if the mean of the measurements is too far away from the prediction (with the distance measured in standard deviations), then we consider the measurements as contradicting the prediction. This makes sense since they fall outside the range of values that could reasonably be expected to occur if the prediction were correct and the standard deviation appropriately quantified. (See prediction interval.)

## Relationship between standard deviation and the mean

The mean and the standard deviation of a set of data are usually reported together. In a certain sense, the standard deviation is a “natural” measure of statistical dispersion if the center of the data is measured about the mean.

## Means and standard deviations

1. Bridget surveyed the price of petrol at petrol stations in Cape Town and Durban. The raw data, in rands per litre, are given below:
 Cape Town 8,96 8,76 9,00 8,91 8,69 8,72 Durban 8,97 8,81 8,52 9,08 8,88 8,68
1. Find the mean price in each city and then state which city has the lowest mean.
2. Assuming that the data is a population find the standard deviation of each city's prices.
3. Assuming the data is a sample find the standard deviation of each city's prices.
4. Giving reasons which city has the more consistently priced petrol?
2. The following data represents the pocket money of a sample of teenagers. 150; 300; 250; 270; 130; 80; 700; 500; 200; 220; 110; 320; 420; 140.What is the standard deviation?
3. Consider a set of data that gives the weights of 50 cats at a cat show.
1. When is the data seen as a population?
2. When is the data seen as a sample?
4. Consider a set of data that gives the results of 20 pupils in a class.
1. When is the data seen as a population?
2. When is the data seen as a sample?

where we get a research paper on Nano chemistry....?
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
Ali
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
hey
Giriraj
Preparation and Applications of Nanomaterial for Drug Delivery
revolt
da
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
ya I also want to know the raman spectra
Bhagvanji
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
Other chapter Q/A we can ask

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