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From Doug Cutting <cutt...@lucene.com>
Subject Re: document & field boosting
Date Mon, 12 Aug 2002 16:33:42 GMT
Clemens Marschner wrote:
> Doug, do you think the ranking function as stated in the FAQ
> (http://lucene.sourceforge.net/cgi-bin/faq/faqmanager.cgi?file=chapter.searc
> h&toc=faq#q31 is still correct after the recent changes?

Yes, this equation is still correct, although it's now incomplete. 
There is now another factor, the boost of the field containing the term, 
specified when that field was indexed.

As I mentioned before, I would eventually like to make it possible for 
folks to easily modify the scoring function.  My idea is to generalize 
the formula to something like:

   sum_t( term_factor(df) * term_doc_factor(tf) * field_factor(length) *
          query_boost * field_boost )

where term_factor(), term_doc_factor() and doc_factor() correspond to 
methods that folks can easily override.

Currently all of the scoring functions are static methods in a single 
class, Similarity.java, so one can in fact currently modify scoring by 
re-defining this class, but it is not well documented and only for the 
brave.

Doug


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