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From "Grant Ingersoll (JIRA)" <>
Subject [jira] Commented: (LUCENE-954) Toggle score normalization in Hits
Date Fri, 22 Feb 2008 11:32:19 GMT


Grant Ingersoll commented on LUCENE-954:

This is why the scores from Hits are currently not comparable to each other

Do you mean across queries or within queries?  Even if you have raw scores, they still won't
be comparable across queries, or at least that is my understanding of the literature.  In
your original case of federated search across several sources, each with their own stats it
is not well understood what scores mean.  Not saying it can't be done, it really is the only
way to do federated search, just not sure one can try to read too much into the scores.  Of
course, this is more of a discussion for the user list than a JIRA issue, so I'd be happy
to discuss more there and hear other thoughts.  It has been a while since I have read anything
on it.

That also isn't to say that your patch isn't worthwhile, just wondering whether the change
is actually meaningful for your use case.

> Toggle score normalization in Hits
> ----------------------------------
>                 Key: LUCENE-954
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Search
>    Affects Versions: 2.2
>         Environment: any
>            Reporter: Christian Kohlschütter
>         Attachments: hits-scoreNorm.patch
> The current implementation of the "Hits" class sometimes performs score normalization.
> In particular, whenever the top-ranked score is bigger than 1.0, it is normalized to
a maximum of 1.0.
> In this case, Hits may return different score results than TopDocs-based methods.
> In my scenario (a federated search system), Hits delievered just plain wrong results.
> I was merging results from several sources, all having homogeneous statistics (similar
to MultiSearcher, but over the Internet using HTTP/XML-based protocols).
> Sometimes, some of the sources had a top-score greater than 1, so I ended up with garbled
> I suggest to add a switch to enable/disable this score-normalization at runtime.
> My patch (attached) has an additional peformance benefit, since score normalization now
occurs only when Hits#score() is called, not when creating the Hits result list. Whenever
scores are not required, you save one multiplication per retrieved hit (i.e., at least 100
multiplications with the current implementation of Hits).

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