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https://issues.apache.org/jira/browse/LUCENE-8563?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16703007#comment-16703007
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Adrien Grand commented on LUCENE-8563:
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My gut feeling is that this change is going to be unnoticed by the vast
majority of users as ordering is preserved. So I'd rather not require changes
on their end to use this simpler implementation of BM25 and just document the
change in runtime behavior in the release notes. I'm happy with keeping Solr on
the current scoring formula and opening a follow-up issue to discuss how to
handle the migration.
[~lucacavanna] Based on Jan's comments, then let's configure Solr's
BM25SimilarityFactory and SchemaSimilarityFactory to use the
LegacyBM25Similarity that you added?
> Remove k1+1 from the numerator of BM25Similarity
> -------------------------------------------------
>
> Key: LUCENE-8563
> URL: https://issues.apache.org/jira/browse/LUCENE-8563
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Adrien Grand
> Priority: Minor
> Time Spent: 40m
> Remaining Estimate: 0h
>
> Our current implementation of BM25 does
> {code:java}
> boost * IDF * (k1+1) * tf / (tf + norm)
> {code}
> As (k1+1) is a constant, it is the same for every term and doesn't modify
> ordering. It is often omitted and I found out that the "The Probabilistic
> Relevance Framework: BM25 and Beyond" paper by Robertson (BM25's author) and
> Zaragova even describes adding (k1+1) to the numerator as a variant whose
> benefit is to be more comparable with Robertson/Sparck-Jones weighting, which
> we don't care about.
> {quote}A common variant is to add a (k1 + 1) component to the
> numerator of the saturation function. This is the same for all
> terms, and therefore does not affect the ranking produced.
> The reason for including it was to make the final formula
> more compatible with the RSJ weight used on its own
> {quote}
> Should we remove it from BM25Similarity as well?
> A side-effect that I'm interested in is that integrating other score
> contributions (eg. via oal.document.FeatureField) would be a bit easier to
> reason about. For instance a weight of 3 in FeatureField#newSaturationQuery
> would have a similar impact as a term whose IDF is 3 (and thus docFreq ~= 5%)
> rather than a term whose IDF is 3/(k1 + 1).
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