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From "zheng" <>
Subject RE: Lucene search formula
Date Fri, 07 Jul 2006 14:36:44 GMT

Can somebody explain the lengthNorm, queryNorm and coord in lucene?
lengthNorm is the (term freq)/(total terms number) or (term freq)/(max term
freq) or something else. queryNorm is the (term squared
weight)/(sumOfSqureWeights)? Why we still need queryNorm when it will not
affect the score for a certain query? How to calculate the coord value?


-----Original Message-----
From: Otis Gospodnetic [] 
Sent: 07 July 2006 06:10
Subject: Re: Lucene search formula

The formula hasn't changed (but the first printing of the book had a portion
of it missing, check javadoc for (Default?)Similarity for the real and
current formula).

Here is a simple IDF example, or at least how I "visualize" IDF.
You have an index with a bunch of documents and terms in it.  A term T can
appear some number of times in this index, say N times.  You can think of
the IDF of the term T is "1/N" (not really 1/N, but....
log(numDocs/(docFreq+1)) + 1).  The more frequent the term in the index, the
smaller its weight (the less important it is during scoring).


----- Original Message ----
From: Rajiv Roopan <>
Sent: Thursday, July 6, 2006 10:46:52 PM
Subject: Lucene search formula

   I was recently looking thru the lucene in action book and came across the
scoring formula. I was wondering if the formula has changed since the book
was written?

  Also was wondering if someone can breifly explain what the IDF(t) term in
the formula means? In the book it says that it's the inverse document
frequency of the term but doesn't explain beyond that?


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