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From "Christoph Goller (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (LUCENE-8000) Document Length Normalization in BM25Similarity correct?
Date Fri, 20 Oct 2017 07:10:00 GMT

     [ https://issues.apache.org/jira/browse/LUCENE-8000?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Christoph Goller updated LUCENE-8000:
-------------------------------------
    Description: 
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. 



  was:
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 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. 




> 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. 



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