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# Standard Deviation??? Watch

1. hey all

The figurs were 230, 700, 600, 600, 1050. I added them up and divided by 58 which gave me my mean (54.82758621).

I need the SD using the n – 1 divisor, of the data.

I know that the answer is "The estimate for the standard deviation is 78.912.. which, to the nearest integer, is 79. "

How the heck is that the answer? I have sat here for ages doing different things, i just cant get that answer. Please help! and then i can do my exam knowing i can do the other questions
2. What?? Why did you divide by 58?? There are only 5 numbers there.. so surely you divide by 5 to get a mean of 636...
3. (sum of x^2 / n) - ((sum of x/n)^2)
root that answer and that will do the trick.
4. You don't divide the mean by n-1. Look at the SD equation again.
5. (Original post by Penguinsaysquack)
What?? Why did you divide by 58?? There are only 5 numbers there.. so surely you divide by 5 to get a mean of 636...
Because the figures i added up were made up of 58 figures, sorry, i didnt explain that bit.

Ik onw for a fact that the mean is correct, as the automated exam said i was right. However I simply cannot figure how to do the SD bit.
6. (Original post by maths134)
(sum of x^2 / n) - ((sum of x/n)^2)
root that answer and that will do the trick.

(Original post by soup)
You don't divide the mean by n-1. Look at the SD equation again.

Im a maths novice, can you please explain to me what i would have to do in order for that formula to work?
7. (Original post by T.I.)
Im a maths novice, can you please explain to me what i would have to do in order for that formula to work?
Draw a table with columns of x and x^2. Where x is your data values (230,700 etc.)
Calculate x^2 by squaring values.
Find sum of each column to get sum of x and sum of x^2.
Enter values into eqn and bam
8. to calculate standard deviation you need variance first. To calculate that you need the equation I posted earlier. Once you have the variance you square root that number to get your variance.

(Square each of your numbers individually) and add them up. Divide this number by the amount of numbers in total. (ill call this P)

Then add all your numbers together and divide by the amount of numbers in total (for the mean) then square this number. (ill call this L)

9. Are you sure you're using the correct formula?

For standard deviation it should be:

Please appreciate that set out like that... it wasn't easy to do

EDIT: I can't seem to make it work.... but either way you can copy the coding:
\Large s=\sqrt{\frac{\Sigma{x-\bar x}^2}{n-1}
into the example spot on the tutorial page for LaTex: http://www.forkosh.com/mimetextutorial.html to see what formula you should be using

EDIT number 2:Nevermind.. managed to make it work
10. (Original post by maths134)
to calculate standard deviation you need variance first. To calculate that you need the equation I posted earlier. Once you have the variance you square root that number to get your variance.

(Square each of your numbers individually) and add them up. Divide this number by the amount of numbers in total. (ill call this P)

Then add all your numbers together and divide by the amount of numbers in total (for the mean) then square this number. (ill call this L)

I just did that and it gave me a random number. argh. i feal like crying!

i dont suppose i can send you a screen shot of the sum i am looking at?
11. Here you go:

12. (Original post by Penguinsaysquack)
Here you go:

I feel so stupid compared to you guys atm, i probably look really thick.

I just dont understand forumulas . I have to have it explained to me in words. I shall say the same to you buddy, is their any chance I can send you a screen pic of the question on sykpe or something? As my figures are from a frequency table so when I try to do it, the answer is never correct.
13. You're probably quite a bit younger than me...
I'll speak the formula then..

Standard deviation is the square root of: the sum of((x-mean)squared) divide by (n-1)
Looks so much more complicated in words...
14. (Original post by Penguinsaysquack)
You're probably quite a bit younger than me...
I'll speak the formula then..

Standard deviation is the square root of: the sum of((x-mean)squared) divide by (n-1)
Looks so much more complicated in words...
I am 20, tbh i'm not really bothered if you are 85. Is there any way i can show you the question please?
15. (Original post by T.I.)
I am 20, tbh i'm not really bothered if you are 85. Is there any way i can show you the question please?
Not really no... Unless you can send me a link to the question if it's online?
16. (Original post by Penguinsaysquack)
Not really no... Unless you can send me a link to the question if it's online?
i will type it:

The question
The sales figures (in millions of US$) of 58 companies are given in the following table: Class Interval 0-20 20-50 50-100 100-200 200-500 Frequency 23 20 8 4 3 Provide the following, giving both your answers (in millions of US$) to the nearest million US\$.

The estimated standard deviation, using the n – 1 divisor, of the data.

The Solution
The frequency table is as follows:
x f
10 23
35 20
75 8
150 4
350 3

The estimate for the mean is 54.827.. which, to the nearest integer, is 55,
The estimate for the standard deviation is 78.912.. which, to the nearest integer, is 79.

The question initialy asked, as you can see for the mean aswell. I managed to get the mean. But i cant at all figure out how to get the answer of 79.

Thanks mate!

Edit: some of the figures arent quite in line, sorry about that but it wont let me line them up on here for some reason.
17. Ok.. I've managed to make the standard deviation = 78.91237377

So first of all you found the mean.. I'll Call it M

so then you do the lengthy calculation of:
((10-M)^2 x 23) + ((35-M)^2 x 20) + ((75-M)^2 x 8) + ((150-M)^2 x 4) + ((350-M)^2 x 3) which equals 354948.2759

You then divide this by 57 (which is n-1) to get 6227.162734

You then square root that to get the answer which is 78.91237377 which is roughly equal to 79

Does that make sense or would you like me to go over any part of that?
18. (Original post by Penguinsaysquack)
Ok.. I've managed to make the standard deviation = 78.91237377

So first of all you found the mean.. I'll Call it M

so then you do the lengthy calculation of:
((10-M)^2 x 23) + ((35-M)^2 x 20) + ((75-M)^2 x 8) + ((150-M)^2 x 4) + ((350-M)^2 x 3) which equals 354948.2759

You then divide this by 57 (which is n-1) to get 6227.162734

You then square root that to get the answer which is 78.91237377 which is roughly equal to 79

Does that make sense or would you like me to go over any part of that?

YES IT WORKS!

Thank you so much.

I didnt think that n-1 meant 58-1.

I can now have another go at my test tommorow, now knowing how to do it (ill have to keep lookiong at this thread tho).

You have earnt a lot of karma tonight. I beleive in it, even if you don't.

I hope you get it back very soon!

thanksss!!
19. (Original post by T.I.)
YES IT WORKS!

Thank you so much.

I didnt think that n-1 meant 58-1.

I can now have another go at my test tommorow, now knowing how to do it (ill have to keep lookiong at this thread tho).

You have earnt a lot of karma tonight. I beleive in it, even if you don't.

I hope you get it back very soon!

thanksss!!
You're welcome

Well I like to believe in karma so I can settle for that

Though I believe in +rep even more
20. I repped you, (i pressed the + button anyway) I think that is how to do it haha

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