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From "Timothy M. Rodriguez (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LUCENE-8000) Document Length Normalization in BM25Similarity correct?
Date Thu, 19 Oct 2017 15:43:00 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-8000?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16211227#comment-16211227
] 

Timothy M. Rodriguez commented on LUCENE-8000:
----------------------------------------------

+1 for keeping the existing behavior of true.  It definitely struck me as weird too, but for
many indexes flipping the default would result in markedly worse behavior.  Rather than disabling
discount overlaps, maybe the more ideal behavior would be making the average document length
equal to the total number of positions across the collection divided by the number of documents?
That way we'd be comparing position length to average position length? However, I haven't
looked into the feasibility or expense of doing that.  If we were able to do that, discountOverlaps
could move to something like countPositions vs countFrequencies.

> 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.
>  {quote} @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)));
>     }
>   }{quote}
> 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.
> {quote}  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);
>     }
>   }{quote}
> 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 our 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. 



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