Christoph Goller created LUCENE-8000:
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Summary: 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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