lucene-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From "Robert Muir (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LUCENE-8000) Document Length Normalization in BM25Similarity correct?
Date Fri, 20 Oct 2017 21:18:00 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-8000?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16213255#comment-16213255
] 

Robert Muir commented on LUCENE-8000:
-------------------------------------

{quote}
Robert Muir thanks for the further explanation. That helped clarify. It does seem the effect
would be minor at best. It'd be an interesting experiment at some point, though. If I ever
get to trying it, I'll post back.
{quote}

Thanks Timothy! Maybe if you get the chance to do the experiment, simply override the method
{{protected float avgFieldLength(CollectionStatistics collectionStats)}} to return the alternative
value. For experiments it can just be a hardcoded number you computed yourself in a different
way.

> Document Length Normalization in BM25Similarity correct?
> --------------------------------------------------------
>
>                 Key: LUCENE-8000
>                 URL: https://issues.apache.org/jira/browse/LUCENE-8000
>             Project: Lucene - Core
>          Issue Type: Bug
>            Reporter: Christoph Goller
>            Priority: Minor
>
> Length of individual documents only counts the number of positions of a document since
discountOverlaps defaults to true.
> {code}
>  @Override
>   public final long computeNorm(FieldInvertState state) {
>     final int numTerms = discountOverlaps ? state.getLength() - state.getNumOverlap()
: state.getLength();
>     int indexCreatedVersionMajor = state.getIndexCreatedVersionMajor();
>     if (indexCreatedVersionMajor >= 7) {
>       return SmallFloat.intToByte4(numTerms);
>     } else {
>       return SmallFloat.floatToByte315((float) (1 / Math.sqrt(numTerms)));
>     }
>   }}
> {code}
> Measureing document length this way seems perfectly ok for me. What bothers me is that
> average document length is based on sumTotalTermFreq for a field. As far as I understand
that sums up totalTermFreqs for all terms of a field, therefore counting positions of terms
including those that overlap.
> {code}
>  protected float avgFieldLength(CollectionStatistics collectionStats) {
>     final long sumTotalTermFreq = collectionStats.sumTotalTermFreq();
>     if (sumTotalTermFreq <= 0) {
>       return 1f;       // field does not exist, or stat is unsupported
>     } else {
>       final long docCount = collectionStats.docCount() == -1 ? collectionStats.maxDoc()
: collectionStats.docCount();
>       return (float) (sumTotalTermFreq / (double) docCount);
>     }
>   }
> }
> {code}
> Are we comparing apples and oranges in the final scoring?
> I haven't run any benchmarks and I am not sure whether this has a serious effect. It
just means that documents that have synonyms or in my use case different normal forms of tokens
on the same position are shorter and therefore get higher scores  than they should and that
we do not use the whole spectrum of relative document lenght of BM25.
> I think for BM25  discountOverlaps  should default to false. 



--
This message was sent by Atlassian JIRA
(v6.4.14#64029)

---------------------------------------------------------------------
To unsubscribe, e-mail: dev-unsubscribe@lucene.apache.org
For additional commands, e-mail: dev-help@lucene.apache.org


Mime
View raw message