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#1 AdamSpeight2008  Icon User is offline

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Question on Standard Deviation

Posted 15 February 2009 - 07:59 AM

Can somebody check my mathematics for me, as i get a different answer depending on which method I used.

By Hand
List = 1,2,3,4,5,6,7

Count=7
Sum
1 + 2=3
3 + 3=6
6 + 4=10
10 + 5=15
15 + 6=21
21 + 7=28

Mean=Sum / Count
=28 / 7
=4

StdDev
=(1-4),(2-4),(3-4),(4-4),(5-4),(6-4),(7-4)
=-3,-2,-1,0,1,2,3
Square Them
=9,4,1,0,1,4,9
Average These
=(9+4+1+1+4+9)/7 == 14*2/7 = 28/7 = 4
Take Positive Square Root
=4^.5
=2

Computer Package

Google Doc Spreadsheet =2.16024689946929
Excel =2.160246899
My Own Written Software = 2
Snippet later

If its correct that its =2 then why are the Professional Packages so wrong?

This post has been edited by AdamSpeight2008: 15 February 2009 - 07:59 AM


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Replies To: Question on Standard Deviation

#2 BigAnt  Icon User is offline

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Re: Question on Standard Deviation

Posted 15 February 2009 - 11:21 AM

Well the variance formula is: [SUM 1-n](xi -xa)^2/(n-1)

and the Standard deviation is the sqrt(variance), so is this the formula you are using, if excel says that then more then likely you made an error in your calculations or entered the data incorrectly.

Looks like you divided by n not n-1
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#3 AdamSpeight2008  Icon User is offline

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Re: Question on Standard Deviation

Posted 15 February 2009 - 12:31 PM

Use this reference StdDev
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#4 AdamSpeight2008  Icon User is offline

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Re: Question on Standard Deviation

Posted 15 February 2009 - 12:44 PM

Work it out there doing population standard deviation
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#5 fuzzylunkinz  Icon User is offline

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Re: Question on Standard Deviation

Posted 15 February 2009 - 01:50 PM

You need to divide by 6 rather than 7. You leave one degree of freedom, hence n-1.
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#6 AdamSpeight2008  Icon User is offline

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Re: Question on Standard Deviation

Posted 15 February 2009 - 02:33 PM

Thank you got it to work out population standard deviation as well.

This post has been edited by AdamSpeight2008: 15 February 2009 - 02:34 PM

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#7 NeoTifa  Icon User is offline

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Re: Question on Standard Deviation

Posted 16 February 2009 - 12:04 PM

This is what I got.... 2.2%

[attachment=10404:attachment]
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#8 KYA  Icon User is offline

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Re: Question on Standard Deviation

Posted 16 February 2009 - 12:37 PM

Your hand writing is terrible :P
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#9 skyhawk133  Icon User is offline

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Re: Question on Standard Deviation

Posted 16 February 2009 - 02:26 PM

I must assume that was done in paint/photoshop and not on a piece of paper then scanned in. :)
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#10 Jayman  Icon User is offline

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Re: Question on Standard Deviation

Posted 16 February 2009 - 05:59 PM

Looks like left handed writing to me.
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#11 NeoTifa  Icon User is offline

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Re: Question on Standard Deviation

Posted 17 February 2009 - 10:47 AM

Shut up. I'm an injiner... :P I used my tablet to scribble that out in MSPaint. :D
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