For more information, please see full course syllabus of Statistics

For more information, please see full course syllabus of Statistics

### Measuring Spread: Range, IQR, Standard Deviation

Range is the simplest way to calculate the spread of a distribution. It is also extremely influenced by outliers.

Before calculating the standard deviation, calculate the variance. Make sure whether you need to find the population variance and standard deviation or a sample variance or standard deviation.

Before calculating the interquartile range, make sure that the data are ordered from lowest to highest.

The standard deviation is the best measure of spreads since it uses all of the observations; however, if the data are skewed, the interquartile range is the preferred measure of the center.

### Measuring Spread: Range, IQR, Standard Deviation

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

- Intro 0:00
- Measuring Spread 0:08
- Range 1:06
- Example
- Example
- Standard Deviation 2:05
- Population Standard Deviation
- Sample Standard Deviation
- Example: Standard Deviation 4:11
- Example: Standard Deviation 6:05
- Interquartile Range (IQR) 8:05
- Example: Interquartile Range 9:03
- Example: Interquartile Range 10:27
- Extra Example 1
- Extra Example 2

0 answers

Post by Barbara garrett on August 13, 2014

Hi Professor Yates,

in the second section of the example how did you get the answer 44 I tried multiplying each number by itself then adding it for example 2^2+ 3^2 and so on and got an answer of 31 not 44 please advise

thank you

0 answers

Post by Unknown Thesisfree on March 5, 2014

GUYS did you know how to get the sample size if the population is infinite? Some one told me that automatically it will become 100 sample size but there is no reference. Is this true?

1 answer

Last reply by: Bora Basa

Fri Aug 23, 2013 11:14 AM

Post by Alex Moon on November 8, 2012

How come for Population Standard deviation, we divide by N, but for Sample Standard Deviation, we divide by n-1? Why is there one less to divide by for samples???

There is no explanation provided.

0 answers

Post by Alex Moon on November 8, 2012

"My IQ are 19.5"

0 answers

Post by harman bhullar on July 30, 2012

Actually if you are not getting the same answers for standard deviation he did, youre probably right. He uses 9 for n instead of 10 in his first standard deviation example. Hope it helps

1 answer

Last reply by: Kirill Lapin

Sat Apr 14, 2012 2:58 PM

Post by Kirill Lapin on April 14, 2012

Using MS Excel for the dataset on the last slide: {27,32,39,44,49,53,57,61}, I am getting the followung results:

fx QUARTILE() Q1=37.25, Q2=46.5, Q3=54

fx QUARTILE.INC() Q1=37.25, Q2=46.5, Q3=54

fx QUARTILE.EXC() Q1=33.75, Q2=46.5, Q3=56

Is it for sure that Q1=Median(lower half) and Q3=Median(upper half) please?

0 answers

Post by Rima Hanhan on March 6, 2012

Hi I need help please. I need to get the Population attributable risk .

I know that the formula is PPoverall -PPunexposed/ppoverall

However, I have only the data of exposed individual to a certain risk factor but I don't have the data of individuals who has the disease and they are unexposed and I don't know if those individuals who are exposed to a certain risk factor has the disease I am interested in.

So how can I compute the attributable risk factor. My purpose is to see how many people who are exposed to this risk factor has a high risk to develop the disease. Please help

0 answers

Post by James Xie on July 30, 2011

Next year I'm taking ap stat.

I'm still concerned on whether we should memorize the std. dev formula or if we could just use what the calculator provides

0 answers

Post by pamela jackson on May 28, 2010

I keep trying to work your samples on standard deviation and cannot arrive at your answers. I do not break down the formula like you--but rather work it out the long way using sumXi-Xbar2. I'm not sure I'm following you two examples up when you "simplify" the formula. For extra example 1, my S = 12.4 (not 10.27). Any insights?

0 answers

Post by Laura Burkett on December 10, 2009

I keep calculating Ex as 146, not 144... is that right?