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From "Joaquin Perez-Iglesias (JIRA)" <>
Subject [jira] Commented: (LUCENE-2091) Add BM25 Scoring to Lucene
Date Tue, 01 Dec 2009 16:02:20 GMT


Joaquin Perez-Iglesias commented on LUCENE-2091:

Hi Otis, Robert and Yuval.

I developed this add-on for Lucene in 2008, for some experiments that I was doing, and I would
like to express my impressions about this. 

In my experience and after reading lot of papers I have never found a case where the Lucene-VSM
implementation improves BM25 performance. 
BM25 (with standard parameters) outperforms Lucene-VSM, moreover a room for improvement exists
if the parameters are fixed specifically for the collection. I made publish some results with
the Eurogov collection some time ago.

I can show you now some experiments with TREC Disk4&5 collection, these results have been
obtained with default parameters with the Robust track topics. As you can see BM25 improves
the Lucene-VSM ranking function.

                 MAP         P@5     
VSM       0.2079     0.4096
BM25     0.2340     0.4578

This implementation is getting more popular and I know that some people is using it on their
research, thus it will be really nice if at some point it is included in the core.

The only concerns that I have about it, are related with:
  - Only simple boolean queries based on terms are supported (with operators or, and, not).
For instance it does not support PhraseQuery.
  - IDF cannot be calculated at a document level (this is important for BM25F).   
  - Another issue is related with computing the document average length, but this could be
easily solved.

These issues are described in detail in the documentation that I made public in my website.

Thanks to all for your interest and work.

Joaquin Perez-Iglesias

> Add BM25 Scoring to Lucene
> --------------------------
>                 Key: LUCENE-2091
>                 URL:
>             Project: Lucene - Java
>          Issue Type: New Feature
>          Components: contrib/*
>            Reporter: Yuval Feinstein
>            Priority: Minor
>             Fix For: 3.1
>         Attachments: persianlucene.jpg
>   Original Estimate: 48h
>  Remaining Estimate: 48h
> describes an implementation of Okapi-BM25 scoring
in the Lucene framework,
> as an alternative to the standard Lucene scoring (which is a version of mixed boolean/TFIDF).
> I have refactored this a bit, added unit tests and improved the runtime somewhat.
> I would like to contribute the code to Lucene under contrib. 

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