[jira] [Created] (MAHOUT-1239) Standardize form of log-likelihood computation

["Sean Owen (JIRA)" <ji...@apache.org>]

Sean Owen created MAHOUT-1239:
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             Summary: Standardize form of log-likelihood computation
                 Key: MAHOUT-1239
                 URL: https://issues.apache.org/jira/browse/MAHOUT-1239
             Project: Mahout
          Issue Type: Improvement
    Affects Versions: 0.7
            Reporter: Sean Owen
            Priority: Minor
             Fix For: 0.8
         Attachments: MAHOUT-1239.patch

qzchenwl@gmail.com reported that LogLikelihood.logLikelihoodRatio() looked like its formula was incorrect, at least with respect to http://tdunning.blogspot.mx/2008/03/surprise-and-coincidence.html

It appears that the calculation is correct but in a different form, that is not immediately recognizable as correct. The proposal here is to change the code to match the blog post and avoid confusion (and ends up avoiding 2 method calls).

(Along the way, I think this fixes a tiny other problem in a related test. We have a test case that detects when round-off would produce a negative LLR and should be clamped to 0, but the test asserts that the result is >0 not >=0.)

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