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From "Robert Muir (JIRA)" <>
Subject [jira] [Commented] (LUCENE-8025) compute avgdl correctly for DOCS_ONLY
Date Tue, 31 Oct 2017 02:36:00 GMT


Robert Muir commented on LUCENE-8025:

I opened this as a separate issue because we may want to backport it. Relevance pretty much
falls apart in this case today with these sims (except Classic which does not use a pivot,
hence unchanged). Here are my results on the bengali collection (I had it handy from working
with the analyzer):

||Sim||Baseline MAP||DOCS_ONLY MAP (master)||DOCS_ONLY MAP (patch)||

I can dig up some other datasets to confirm.

> compute avgdl correctly for DOCS_ONLY
> -------------------------------------
>                 Key: LUCENE-8025
>                 URL:
>             Project: Lucene - Core
>          Issue Type: Bug
>            Reporter: Robert Muir
> Spinoff of LUCENE-8007:
> If you omit term frequencies, we should score as if all tf values were 1. This is the
way it worked for e.g. ClassicSimilarity and you can understand how it degrades. 
> However for sims such as BM25, we bail out on computing avg doclength (and just return
a bogus value of 1) today, screwing up stuff related to length normalization too, which is
> Instead of a bogus value, we should substitute sumDocFreq for sumTotalTermFreq (all postings
have freq of 1, since you omitted them).

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