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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-1534) idf(t) is not actually squared during scoring?
Date Mon, 02 Feb 2009 21:40:00 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-1534?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12669750#action_12669750
] 

Doug Cutting commented on LUCENE-1534:
--------------------------------------

I've always found "idf squared" an unhelpful description.  We're computing a dot-product of
two vectors, the angle between them.  Terms are dimensions.  The magnitude in each dimension
is the weight of the term in a query or document.  Our heuristic for computing weights is
(sqrt(tf)*idf)/norm.  Put all that together, and you do indeed get an "idf squared" factor
in each addend of the score.  But if we feel that over-emphasizes terms with large idfs, then
we should not remove an idf factor from one vector, but rather rework our weight heuristic,
perhaps replacing idf with sqrt(idf), no?

> idf(t) is not actually squared during scoring?
> ----------------------------------------------
>
>                 Key: LUCENE-1534
>                 URL: https://issues.apache.org/jira/browse/LUCENE-1534
>             Project: Lucene - Java
>          Issue Type: Bug
>          Components: Query/Scoring
>    Affects Versions: 2.1, 2.2, 2.3, 2.3.1, 2.3.2, 2.4
>            Reporter: Michael McCandless
>            Assignee: Michael McCandless
>            Priority: Minor
>             Fix For: 2.9
>
>
> The javadocs for Similarity:
>   http://lucene.apache.org/java/2_4_0/api/org/apache/lucene/search/Similarity.html
> show idf(t) as being squared when computing net query score.  But I
> don't think it is actually squared, in looking at the sources?  Maybe
> it used to be, eg this interesting discussion:
>   http://markmail.org/message/k5pl7scmiac5wosb
> Or am I missing something?  We just need to fix the javadocs to take
> away the "squared"...

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