## Standard Deviation Help

Debate and discussion of any biological questions not pertaining to a particular topic.

transistor
Garter
Posts: 1
Joined: Sun Jan 04, 2009 11:53 pm

### Standard Deviation Help

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
Viper
Posts: 672
Joined: Fri Mar 30, 2007 4:23 am

### Re: Standard Deviation Help

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
Garter
Posts: 1
Joined: Fri Jan 09, 2009 12:47 am

### Re: Standard Deviation Help

Another webpage that might help:

standard deviation

GreenDog
Coral
Posts: 126
Joined: Sun Mar 27, 2005 8:37 am
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

stereologist
Garter
Posts: 8
Joined: Sat Dec 13, 2008 12:20 pm
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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