[jira] Commented: (MAHOUT-389) UncenteredCosineSimilarity

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

    [ https://issues.apache.org/jira/browse/MAHOUT-389?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12863861#action_12863861 ] 

Sean Owen commented on MAHOUT-389:
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The issue with that is you are implicitly assuming that missing ratings are 0, and that's not a good assumption. For example, if your ratings are on a scale of 1 to 5, you are assuming that all unknown items are utterly hated.

It is not ignoring zeroes; it is ignoring missing ratings.

I think the distributed computation should act the same way for similar reasons.

> UncenteredCosineSimilarity 
> ---------------------------
>
>                 Key: MAHOUT-389
>                 URL: https://issues.apache.org/jira/browse/MAHOUT-389
>             Project: Mahout
>          Issue Type: Bug
>          Components: Collaborative Filtering
>            Reporter: Sebastian Schelter
>         Attachments: MAHOUT-389.patch
>
>
> org.apache.mahout.cf.taste.impl.similarity.UncenteredCosineSimilarity only computes the cosine distance between those components of the vectors where both vectors have a value greater zero.
> This is inconsistent with the definition of the cosine (correct me if I'm wrong) and is inconsistent with the distributed cosine similarity computation.

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