# Standard Deviation in Excel, SigmaPlot and SPSS

#### Lukas

##### New Member
Hi all,

I have found this forum through a google search and this is my first post. I usually read and ocassionally post on an astronomy forum (http://www.bautforum.com/). I have also asked this question on their Q&A section.
I thought that maybe there might be more specialized knowledge on this forum regarding this particular subject.
Thank you in advance for taking time to ponder my questions.

I have read that the way Excel calculates standard deviation
=STDEV(A1:A9)
is not "proper". This function uses the formula
stdev = sqrt( (n * SUMof (samples^2) - SUMof(samples)^2) / (n* (n-1) )
In excel's help it is described as the "unbiased" method.

The "proper" way according to some sources is:
stdev = sqrt( 1/n SUMof(( sample - mean)^2) )
This can be achieved in Excel by using
=STDEVP(A1:A9)
I've also read that excel should not be used for statistical purposes at all. Now I am being introduced to actual statistical packages such as Sigmalpot and SPSS. And I've found that they calculate standard deviation the "Excel way" (at least in the cases I have observed).

Should I be concerned?
I've got a feeling that both formulas might have their merits but should be used under different circumstances. If this is right, when should I use which formula?

I did some reading but haven't really reached a conclusion for myself yet. Here are some of the links I've looked at:

http://www.maryparker.org/excel/stdev.htm
http://www.cs.uiowa.edu/~jcryer/JSMTalk2001.pdf
http://en.wikipedia.org/wiki/Standard_deviation (plus its talkpage)