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Bhappy 01-21-2009 05:01 AM

Standard deviation: help me in calculating it
 
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.

NATHANALANPATRICKROGERS 01-25-2009 03:03 PM

Re: Standard deviation: help me in calculating it
 
Quote:

Originally Posted by Bhappy (Post 262035)
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.

1. Most scientific calculators can compute the SD of a given set of numbers.
2. MS excel has a number of useful functions, the main being STDEV.
3. Wiki believes 'if many data points are close to the mean, then the standard deviation is small; if many data points are far from the mean, then the standard deviation is large. If all data values are equal, then the standard deviation is zero.'
4. If your teacher wants you to work through the data analysis yourself, then forget all I have said, sharpen your pencil, and consult at length with a quality text book [if you can find one that isn't written by socialist-types :xmas:].

hokie 01-26-2009 01:22 PM

Re: Standard deviation: help me in calculating it
 
SD is a measure of dispersion. Here is how to do it by hand.

1. Collect all of your numbers. There must be at least 2 or the answer is 0.
2. Average all of the numbers: add up and divide by n
3. Subtract the mean from each number
4. Square the differences between the mean and the numbers
5. Add up all of the squares and divide by n-1, not n
6. Take the square root

Suppose the numbers were tree heights in meters
1.
2. in meters
3. in meters
4. meters squared
5. meters squared
6. meters

So SD is in the same units as the original measurements.

Caveat. If you have collected all of the possible measurements such as the heights of all trees then in step 5 divide by n, not n-1.

Suppose you wanted to know the SD of the trees in a forest. If you measured all trees, then the number is not an estimate so you divide by n. If you measured only a subset of trees, a sample, then the SD is an estimate of the SD for the forest and you divide by n-1.

hokie 01-26-2009 01:23 PM

Re: Standard deviation: help me in calculating it
 
SD is a measure of dispersion. Here is how to do it by hand.

1. Collect all of your numbers. There must be at least 2 or the answer is 0.
2. Average all of the numbers: add up and divide by n
3. Subtract the mean from each number
4. Square the differences between the mean and the numbers
5. Add up all of the squares and divide by n-1, not n
6. Take the square root

Suppose the numbers were tree heights in meters
1.
2. in meters
3. in meters
4. meters squared
5. meters squared
6. meters

So SD is in the same units as the original measurements.

Caveat. If you have collected all of the possible measurements such as the heights of all trees then in step 5 divide by n, not n-1.

Suppose you wanted to know the SD of the trees in a forest. If you measured all trees, then the number is not an estimate so you divide by n. If you measured only a subset of trees, a sample, then the SD is an estimate of the SD for the forest and you divide by n-1.

MoleculeInAction 02-04-2009 08:12 AM

Re: Standard deviation: help me in calculating it
 
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