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https://issues.apache.org/jira/browse/LUCENE-8000?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16211193#comment-16211193
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Robert Muir commented on LUCENE-8000:
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I don't think we should disable discountOverlaps:
The reason is that there are too many commonly-used tokenfilters adding
synonyms or similar and they will bias document lengths. I've done measurements
here, and that's why i originally proposed enabling it by default (the option
was there, but was disabled by default).
average document length will never be exact either (due to deleted documents
and many other reasons). norm is inexact since its a single byte too.
Ultimately this average is just a pivot, it doesn't need to be pedantically
correct. and we shouldn't make relevance worse for no good reason.
if you have a different/special use-case, you can disable discountOverlaps
yourself, that's why the option is there.
> 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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