[
https://issues.apache.org/jira/browse/MAHOUT-898?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13158668#comment-13158668
]
Paulo Villegas commented on MAHOUT-898:
---------------------------------------
|When you do test with log-likelihood, make sure you try with two strategies.
First with only positive |votes as interactions and secondly with any vote as
an interaction.
I'll do that, yes. Can't work on it right now, but will try it later this week.
Sean: yes, that would be a good solution. Anyone would then be able to try both
approaches for their use case (since I don't believe there is such a thing as a
universal solution).
> Error in formula for preference estimation in GenericItemBasedRecommender
> -------------------------------------------------------------------------
>
> 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
> Labels: patch
> Fix For: 0.6
>
> Attachments: GenericItemBasedRecommender.diff
>
>
> 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
--
This message is automatically generated by JIRA.
If you think it was sent incorrectly, please contact your JIRA administrators:
https://issues.apache.org/jira/secure/ContactAdministrators!default.jspa
For more information on JIRA, see: http://www.atlassian.com/software/jira
