Quick question about finding standard deviation

  • #1
rock.freak667
Homework Helper
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[SOLVED] Quick question about finding standard deviation

Now I know that [tex]\sigma=\sqrt{var(x)}[/tex]

which simplifies to this expression : [tex]\sigma=\sqrt{\frac{1}{N}\sum_{i=1}^{N}(x-\overline{x})^2}[/tex] can someone show me how they got such an expression?

and in chemistry I have to use a standard deviation calculation to get out a problem. Now normally I would use the above equation but my notes tell me to use this equation:

[tex]\sigma=\sqrt{\frac{1}{N-1}\sum_{i=1}^{N}(x-\overline{x})^2}[/tex]


Which one is correct to use? and can someone tell me if this is correct [tex]c_v =\frac{\sigma}{\overline{x}}[/tex] where [tex]c_v[/tex] is the coefficient of variation
 

Answers and Replies

  • #2
mathman
Science Advisor
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The definition of the st. dev. depends on whether or not the mean is the true mean or the average of the experimental data. For the true mean use 1/N, for the experimental average use 1/(N-1). If you take the average of the variance with the experimental mean and compare it to the average of the variance with the true mean, you will see they are equal.
 
  • #3
rock.freak667
Homework Helper
6,230
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Oh I see now,thank you
 

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