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From Rads2029 <radh...@gmail.com>
Subject Re: Modifying score based on tf and slop
Date Mon, 06 Jul 2009 13:10:04 GMT

Thanks , That helped clear quite a few things.

A few questions though :

1) Regarding tf not making a difference : I do believe that override tf to
return 1 makes a difference.

When I did not override tf the score on doc(AB BC BC CD) was higher on doc (
AB BC CD)
When I did not override tf the score on doc(AB BC xx xx CD) was lesser than
the score on doc ( AB BC CD)

When I overrode tf to return 1 both doc( AB BC BC CD) and ( AB BC CD) had
the same score. 
When I overrode tf to return 1 both doc( AB BC xx xx CD) and ( AB BC CD) had
the same score. 

I do want doc( AB BC BC CD) and doc( AB BC CD) to show me same score but I
also want score of 
doc( AB BC xx xx CD) to be less than score of doc( AB BC CD) .

Also in the score() method of the SpanScorer class , this is the code:

public float score() throws IOException {
    float raw = getSimilarity().tf(freq) * value; // raw score
    return norms == null? raw : raw * Similarity.decodeNorm(norms[doc]); //
normalize
  }

As you can see, tf is used  here

2)" if you really just want to know about the lengths of instances of Spans
in your 
index, you can call the getSpans method directly on your SpanNearQuery and 
iterate over them yourself, ignoring the ones you want to ignore"

Could you throw more light on this? How exactly would I know which ones are
the spans which I need to ignore

2) "subclass SpanQuery so it returns a new NearSpansNoOverlapping that you
would have wrap the NearSpansOrdered and only return the "shortest" span
from each doucment."

Please give me some more details on how to go about this?


Thanks again a lot for ur help.

hossman wrote:
> 
> 
> (Disclaimer: i'm not currently looking at the code, this email is entirely 
> a guess based on what i remember about SpanQueries)
> 
> : II ) Using default implementation of tf in Similarity class:
> : 
> : Case 1 -  Doc : "AB BC BC CD"
> : Result :  4  - Actual score
> : % match :  ( actual score / max possible score) =  ( 4/3) > 100% - This
> is
> : Wrong as I dont want score to be affected by no of times BC occurs
> 
> I suspect you are missunderstanding why you are getting the scores you are 
> getting.
> 
> if i remember correctly, SpanNearQuery ignores all score information 
> coming from the sub-queries it contains and only scores documents based on 
> the distances of the matching Spans (this is true for all of hte 
> "container" span queries i believe - because they all use SpanScorer does 
> and it *only* looks at the Spans)
> 
> So i don't think anything in your SpanNearQuery is actually rewarding a 
> doc for matching one of the individual terms more then once, because 
> nothing ever looks at the tf() of the individual terms.  (if you use a 
> custom Similarity, and override the tf(int) method to include some 
> logging, i'm 90% certain you'll see that that method never get called with 
> any SpanQuery)
> 
> SpanScorer *does* look at every matching Span in a document however -- and 
> assuming you are allowing slop (and it appears you are since other 
> examples you list depend on it) the sequence "AB BC CD" exists twice in 
> your example document above -- once using the BC at position 2, and once 
> using the BC at position 3 - hence the higher then (you) expected score.  
> (if you use a custom Similarity, and override the tf(float) method to 
> include some logging, i'm 90% certain you'll see that that method get  
> called twice for that span query against an index with only that document 
> -- once per instance of the span.
> 
> I'm fairly certain that finding overlapping spans is considered a 
> "feature" of SpanQuery.  I suspect if you look through the test cases for 
> SpanNearQuery you'll even find some examples just like yours where it 
> requires that their be multiple matches.
> 
> 
> looking at the online javadocs, i don't see any simple option to prevent 
> overlapping spans when constructing the SpanNearQuery, but i think it 
> would be fairly easy for you to subclass SpanQuery so it returns a new 
> NearSpansNoOverlapping that you would have wrap the NearSpansOrdered and 
> only return the "shortest" span from each doucment.
> 
> Incidently: if you find subclassing SpanNearQuery tedious to do what you, 
> keep in mind that you don't have to go use IndexSearcher and and deal with 
> the normal scoring (Using slopyFreq & tf) if you don't wnat to -- if you 
> really just want to know about the lengths of instances of Spans in your 
> index, you can call the getSpans method directly on your SpanNearQuery and 
> iterate over them yourself, ignoring the ones you want to ignore.
> 
> 
> 
> -Hoss
> 
> 
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