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Subject Re: tf*idf scoring
Date Tue, 03 Nov 2009 13:54:03 GMT
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> >
> >
> > According to different algorithms, the tf for term c would be 3 / 1 =
> > 0.33 instead of 1 returned by Solr.
>
> I don't follow.  The TF (term frequency) is the number of times the
> term c occurs in that particular document, i.e. 1 time.

I see that above, and below, i made some typo's.  I wrote 3 / 1 = 0.3
instead of 1 / 3 = 0.33. Term c has a #occurences of 1 which the other
algorithms normalize by dividing by the number of terms. So instead of a
tf = #occurences (1) other algorithms do tf = #occurences / #terms
(0.33).

>
> > Also, the tf*idf value i get is 0.5
> > for term c and i get 0.333 for term a. It looks like tf*idf is
> > quotient
> > of document frequency and term frequency.
>
> Yes, indeed.  IDF == Inverse Document Frequency, in other words, 1/DF.

Indeed, but most algorithms i have seen on this topic calculate idf by
ln(#docs / df), this is also true for Lucene as i read
http://lucene.apache.org/java/2_9_0/api/core/org/apache/lucene/search/Similarity.html

idf(t)  =   1 + log (numDocs / df + 1)

>
> >
> > If i calculate tf*idf, for term c in the first document, according to
> > other algorithms it would be:
> >
> > tf = 3 / 1 = 0.333
>
> 3/1 = 3, no?  I don't see where in your docs above you could even get
> a 3 for the letter c.

Here's the other typo, i wrote again 3 / 1 = 0.33 what should've been
1 / 3 = 0.33, of course. The differences i see are:

tf (solr) = #occurences_of_term_T in document_D
tf (other) = #occurences_of_term_T in document_D / #terms_document_D

df (solr) = #occurences_of_term_T in all_documents
df (other) = #occurences_of_term_T in all_documents

idf (solr) = tf / df
idf (other) = ln(#documents / df)

tf*idf (solr) = tf / df
tf*idf (other) = tf * idf

>
> > idf = ln(6 / 3) = 1.0986
> > tf*idf = 0.333 * 1.0986 = 0.3658
> >
>
> I think the formulas you are looking at are doing operations to
> normalize the values, whereas the Solr/Lucene stuff above is telling
> you their raw values.  Note, Lucene/Solr does length normalization,
> etc. too, it just isn't encoded into the TF or DF.  For more on
> Lucene's scoring, see http://lucene.apache.org/java/2_9_0/scoring.html
>

I see, but why not return the true values of Lucene? I did not
reconfigure Solr's scheme to use another algorithm for similarity and
the above Lucene similarity docs state that they use similar
calculations as i have in DefaultSimilarty.

> --------------------------
> Grant Ingersoll
> http://www.lucidimagination.com/
>
> Search the Lucene ecosystem (Lucene/Solr/Nutch/Mahout/Tika/Droids)
> using Solr/Lucene:
> http://www.lucidimagination.com/search
>

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