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Hi folks,

I plugged the following set of numbers into Excel:

[600 470 170 430 300]

I found the mean (394), summed the numbers, divided by 5 to get variance (21704). I then did the square root to find stdev (147.32).

Using Excel or Wolfram Alpha's variance and stdev function give me differnt numbers, variance 27130 and stdev 164.71 respectively. Why? I believe I am following the equation as listed on Wikipedia, my stat text book, and the following page:

http://www.mathsisfun.com/data/standard-deviation.html

What am I missing?

Thank you for any help you can give!

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Maybe math.stackexchange.com/questions/15098/… can help you. –  Shai Covo Feb 4 '11 at 0:38
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closed as off topic by Andrey Rekalo, Dmitri Pavlov, Tim Perutz, David Roberts, Yemon Choi Feb 4 '11 at 1:06

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2 Answers

up vote 0 down vote accepted

The unbiased variance estimator normalizes by $n-1$, instead of $n$. This may be the difference you're seeing.

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You found the population standard deviation; Excel/Wolfram gave the sample standard deviation--the unbiased estimate of the population standard deviation when you have only a sample, not the entire population. Use "stdevp" to get the population standard deviation in Excel.

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