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).
Otis
 Original Message 
From: Rajiv Roopan <rajiv.roopan@gmail.com>
To: javauser@lucene.apache.org
Sent: Thursday, July 6, 2006 10:46:52 PM
Subject: Lucene search formula
Hello,
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?
thanks,
rajiv

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