Standard Deviation Help

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transistor
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Standard Deviation Help

Post by transistor » Mon Jan 05, 2009 12:00 am

Hiya,

I need to calculate standard deviation for a biology test report. Does anyone know any websites with a step by step example of calculating standard deviation? I've had trouble finding anything for people who aren't good at math.

Cheers.

blcr11
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Re: Standard Deviation Help

Post by blcr11 » Mon Jan 05, 2009 1:31 am

http://hubpages.com/hub/deviation

It's step-by-step, anyway. Basically, calculate the mean of the observations. Subtract the mean from each of the observations (= calculate the deviations from the mean). Square the deviations. Sum up the squared deviations. Divide this sum by #Observations - 1 (N-1) and take the square root of the dividend and you've got the standard deviation of the sample.

chairlift
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Re: Standard Deviation Help

Post by chairlift » Fri Jan 09, 2009 12:52 am

Another webpage that might help:

standard deviation

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GreenDog
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Post by GreenDog » Tue Jan 13, 2009 2:38 pm

Why working when someone already did this for you?
After finishing the course in math in the firs year I solemnly swore not to make another calculation again. Enter your data in Excel, and use the STDEV function.
Same goes for any other calculation you have to make, including protein amounts for western.
"When In Danger Or In Doubt Run In Circles Scream And Shout"
Lawrence J. Peter

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stereologist
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Post by stereologist » Wed Jan 21, 2009 5:45 am

There is a small caveat here.
The standard deviation of the sample uses 1/(N-1).
The standard deviation of the population uses 1/N.

The reason for this difference is that 1/(N-1) results in an unbiased estimate of the standard deviation of the population.

Typically you do not do an experiment which generates exhaustive results. So you have a sample and use 1/(N-1).

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