[jira] [Created] (MAHOUT-1144) Wrong normalization in SVD++

["Sebastian Schelter (JIRA)" <ji...@apache.org>]

Sebastian Schelter created MAHOUT-1144:
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             Summary: Wrong normalization in SVD++
                 Key: MAHOUT-1144
                 URL: https://issues.apache.org/jira/browse/MAHOUT-1144
             Project: Mahout
          Issue Type: Bug
          Components: Collaborative Filtering
    Affects Versions: 0.8
            Reporter: Sebastian Schelter
            Assignee: Sean Owen


Reported by Agnonchik:

Seems that I have found a discrepancy of this implementation from the original Yehuda Koren's SVD++ algorithm.

line 140:
double denominator = Math.sqrt(itemsByUser.size());
should be
double denominator = Math.sqrt(itemsByUser.get(u).size());
line 164:
double denominator = Math.sqrt(itemsByUser.size());
should be
double denominator = Math.sqrt(itemsByUser.get(u).size());


The sum of y parameters should be normalized by square root of number of items for which user u provided implicit feedback. Am I right?
Currently, it is normalized by square root of number of users not items.

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