# Question on Standard Deviation

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## 10 Replies - 920 Views - Last Post: 17 February 2009 - 10:47 AM

• MrCupOfT

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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

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

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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

• MrCupOfT

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## Re: Question on Standard Deviation

Posted 15 February 2009 - 12:31 PM

Use this reference StdDev

• MrCupOfT

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## Re: Question on Standard Deviation

Posted 15 February 2009 - 12:44 PM

Work it out there doing population standard deviation

### #5 fuzzylunkinz

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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.

• MrCupOfT

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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

### #7 NeoTifa

• Whorediot

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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]

### #8 KYA

• g++ jameson.cpp -o beverage

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## Re: Question on Standard Deviation

Posted 16 February 2009 - 12:37 PM

### #9 skyhawk133

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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.

### #10 Jayman

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## Re: Question on Standard Deviation

Posted 16 February 2009 - 05:59 PM

Looks like left handed writing to me.

### #11 NeoTifa

• Whorediot

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## Re: Question on Standard Deviation

Posted 17 February 2009 - 10:47 AM

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