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Commons Math - Variance

Posted on 2011-09-21
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The attached spreadsheet has a dataset of: 2,4,6,8,10,12
The following excel functions give results:

VAR:      14
VARP:      11.66666667

Then In Java, if I code like so:
org.apache.commons.math.stat.descriptive.moment.Variance variance = new Variance();
double sigma = variance.evaluate(myDoubleArray);

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where myDoubleArray contains 2.0, 4.0, 6.0, 8.0, 10.0, 12.0
then sigma is 14.0.
So I would conclude that the variance calculated by Commons Math is a sample variance not a population variance.

How would I calculate a sample variance using Commons Math?



standardDeviation.xlsx
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Question by:allelopath
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by:sdstuber
ID: 36576153
var =     (sum(  x - M)^2) / (n-1)
varp =  (sum(  x - M)^2) / n

where M  = Mean of the x population
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by:allelopath
ID: 36576246
The question is:
How would I calculate a sample variance using Commons Math?
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sdstuber earned 500 total points
ID: 36576310
just apply the formulas above...

if VAR is sum of squares divided by n-1, then multiply by n-1 and then divide by n to get VARP


variance.evaluate(myDoubleArray) * ( myDoubleArray.length - 1) / myDoubleArray.length
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Expert Comment

by:aburr
ID: 36576322
M is the mean of the sample (the mean of the populataion being unknown
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