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From "Mark Miller (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-1896) Modify confusing javadoc for queryNorm
Date Thu, 10 Sep 2009 02:01:57 GMT
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[ https://issues.apache.org/jira/browse/LUCENE-1896?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12753377#action_12753377
]

Mark Miller commented on LUCENE-1896:
-------------------------------------

Okay - think I was a tad off base -

Here is the cosine def used:

{code}
cos(a) = V(q) dot V(d) /  |V(q)||V(d)|
{code}

So the cosine is the query vector dot the document vector divided by the magnitude of the
vectors. Classically, |V(q)||V(d)| is a normalization factor that takes the vectors to unit
vectors (so you get the real cosine)

{code}
cos(a) = v(q) dot v(d)
{code}

This is because the magnitude of a unit vector is 1 be definition.

But we don't care about absolute numbers, just relative numbers (as has been often pointed
out) - so the IR guys already fudge this stuff.

While I thought that the queryNorm correlates to |V(q)||V(d)| before, I was off - its just
|V(q)|.  |V(d)| is replaced with the document length normalization, a much faster calculation
with similar properties - a longer doc would have a larger magnitude most likely.

So one strategy is just to not normalize query - though the lit i see doing this is very inefficiently
calculating the query norm in the inner loop - we are not doing that, and so its not much
of an optimization for us.

{code}
cos(a) = V(q) dot V(d) /  |V(d)| == cos(a) * |V(q)| = v(q) dot v(d)
{code}

And it does make queries less comparable (an odd goal I know, but for free?) ;)

Sorry I was a little off earlier - just tried to learn all this myself - and linear alg was
years ago - and open book tests lured my younger, more irresponsible self to not go to the
classes ...

Anyhow, thats my current understanding - please point out if you know I have something wrong.

> Modify confusing javadoc for queryNorm
> --------------------------------------
>
>                 Key: LUCENE-1896
>                 URL: https://issues.apache.org/jira/browse/LUCENE-1896
>             Project: Lucene - Java
>          Issue Type: Improvement
>            Reporter: Jiri Kuhn
>            Priority: Minor
>             Fix For: 2.9
>
>
> See http://markmail.org/message/arai6silfiktwcer
> The javadoc confuses me as well.

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