[jfreechart-dev] [ jfreechart-Bugs-2036061 ] Standard deviation not calculated correctly

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Bugs item #2036061, was opened at 2008-08-02 13:45
Message generated for change (Comment added) made by c_katz09
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Category: General
Group: 1.0.x
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Nobody/Anonymous (nobody)
Assigned to: Nobody/Anonymous (nobody)
Summary: Standard deviation not calculated correctly

Initial Comment:
Maybe this is already reported but
Statistics.getStdDev() gives wrong results because of
the last line of the method:

return Math.sqrt(sum / (data.length - 1));

You should not subtract 1 from data.length.
Thanks, nice software.

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Comment By: Christopher Katz (c_katz09)
Date: 2008-12-01 23:28

Message:
I agree with your point, if we can state with certainty that the data array
is the entire population of data that the standard deviation is calculated
for. If we are only getting data on a sample of a population, a more
accurate estimate of the standard deviation of the entire population
subtracts 1 from the length of the array. But would there be a case when
the method is only passed a sample of data instead of the entire population
of data?

This link gives a good description of the differences in standard
deviation calculations:
http://www.graphpad.com/faq/viewfaq.cfm?faq=1382

Regardless, IMO subtracting 1 is generally accurate in all cases, so the
code should be kept the way it is

- Chris

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