AbstractJDBCDiffStorage.updateItemPref is updating the AVG incorrectly in most
cases
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Key: MAHOUT-576
URL: https://issues.apache.org/jira/browse/MAHOUT-576
Project: Mahout
Issue Type: Bug
Components: Collaborative Filtering
Affects Versions: 0.4
Reporter: Renaud Bruyeron
Fix For: 0.5
the JDBC version of the DiffStorage is not using a RunningAverage in the
removePreference case, and ends up making incorrect calculations.
In a scenario where users are setting and removing a lot of preferences, the
AVG stored in the diff table quickly diverges from the correct value because of
this.
Right now, the input to updateItemPref comes from SlopeOneRecommender, and in
the case of removePreference, it is *the old preference* value, not a delta.
However, the code uses it as if it were a delta. Thus the calculation is off by
PEER(removedpreference,userid)/count everytime a user removes a preference.
At first glance, the code should compute the old delta instead of the old
preference, and use this in the updateItemPref
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