Error in formula for preference estimation in GenericItemBasedRecommender
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Key: MAHOUT-898
URL: https://issues.apache.org/jira/browse/MAHOUT-898
Project: Mahout
Issue Type: Bug
Components: Collaborative Filtering
Environment: mahout-core
Reporter: Paulo Villegas
Assignee: Sean Owen
Priority: Minor
Fix For: 0.6
The formula to estimate the preference for an item in the Taste item-based
recommender normalizes by the sum of similarities for items used in estimation.
But the terms in the sum taken to normalize should be in absolute value, since
they can be negative (e.g. when using Pearson correlation, similarity is in
[-1,1]). Now they are not, and as a result when there are negative and positive
values they cancel out, giving a small denominator and incorrectly boosting the
preference for the item (symptom: it is easy for a predicted preference to take
the maximum value, since the quotient becomes large and it is capped afterwards)
The patch is rather trivial (a one-liner, actually) for
src/main/java/org/apache/mahout/cf/taste/impl/recommender/GenericItemBasedRecommender.java
Note: the same error & suggested fix happens in GenericUserBasedRecommender
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