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https://issues.apache.org/jira/browse/MATH-323?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12794740#action_12794740
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Phil Steitz commented on MATH-323:
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The definition provided here: http://www.jstor.org/pss/2330500 makes sense to
me as a "semivariance" measure. An unbiased estimator would be the sum of the
squared deviations from the target of the values below the target divided by
the total number of values.
> Add Semivariance calculation
> ----------------------------
>
> Key: MATH-323
> URL: https://issues.apache.org/jira/browse/MATH-323
> Project: Commons Math
> Issue Type: New Feature
> Affects Versions: 2.1
> Reporter: Larry Diamond
> Assignee: Phil Steitz
> Priority: Minor
> Fix For: 2.1
>
> Attachments: patch.txt, patch2.txt, StatUtils.java, StatUtils.java,
> StatUtilsTest.java, StatUtilsTest.java
>
>
> I've added semivariance calculations to my local build of commons-math and I
> would like to contribute them.
> Semivariance is described a little bit on
> http://en.wikipedia.org/wiki/Semivariance , but a real reason you would use
> them is in finance in order to compute the Sortino ratio rather than the
> Sharpe ratio.
> http://en.wikipedia.org/wiki/Sortino_ratio gives an explanation of the
> Sortino ratio and why you would choose to use that rather than the Sharpe
> ratio. (There are other ways to measure the performance of your portfolio,
> but I wont bore everybody with that stuff)
> I've already got the coding completed along with the test cases and building
> using mvn site.
> The only two files I've modified is
> src/main/java/org/apache/commons/stat/StatUtils.java and
> src/test/java/org/apache/commons/math/stat/StatUtilsTest.java
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