Package org.apache.lucene.search.similarities

Class DFRSimilarity

java.lang.Object

org.apache.lucene.search.similarities.Similarity

org.apache.lucene.search.similarities.SimilarityBase

org.apache.lucene.search.similarities.DFRSimilarity

public class DFRSimilarity
extends SimilarityBase

Implements the divergence from randomness (DFR) framework introduced in Gianni Amati and Cornelis Joost Van Rijsbergen. 2002. Probabilistic models of information retrieval based on measuring the divergence from randomness. ACM Trans. Inf. Syst. 20, 4 (October 2002), 357-389.

The DFR scoring formula is composed of three separate components: the basic model, the aftereffect and an additional normalization component, represented by the classes BasicModel, AfterEffect and Normalization, respectively. The names of these classes were chosen to match the names of their counterparts in the Terrier IR engine.

To construct a DFRSimilarity, you must specify the implementations for all three components of DFR:

1. BasicModel: Basic model of information content:
   • BasicModelBE: Limiting form of Bose-Einstein
   • BasicModelG: Geometric approximation of Bose-Einstein
   • BasicModelP: Poisson approximation of the Binomial
   • BasicModelD: Divergence approximation of the Binomial
   • BasicModelIn: Inverse document frequency
   • BasicModelIne: Inverse expected document frequency [mixture of Poisson and IDF]
   • BasicModelIF: Inverse term frequency [approximation of I(ne)]
2. AfterEffect: First normalization of information gain:
   • AfterEffectL: Laplace's law of succession
   • AfterEffectB: Ratio of two Bernoulli processes
   • AfterEffect.NoAfterEffect: no first normalization
3. Normalization: Second (length) normalization:
   • NormalizationH1: Uniform distribution of term frequency
   • NormalizationH2: term frequency density inversely related to length
   • NormalizationH3: term frequency normalization provided by Dirichlet prior
   • NormalizationZ: term frequency normalization provided by a Zipfian relation
   • Normalization.NoNormalization: no second normalization

Note that qtf, the multiplicity of term-occurrence in the query, is not handled by this implementation.

See Also:

• BasicModel
• AfterEffect
• Normalization

Nested Class Summary

Nested classes/interfaces inherited from class org.apache.lucene.search.similarities.Similarity

Similarity.SimScorer, Similarity.SimWeight

Field Summary

Fields

Modifier and Type	Field	Description
protected final AfterEffect	afterEffect	The first normalization of the information content.
protected final BasicModel	basicModel	The basic model for information content.
protected final Normalization	normalization	The term frequency normalization.

Fields inherited from class org.apache.lucene.search.similarities.SimilarityBase

discountOverlaps

Constructor Summary

Constructors

Constructor	Description
DFRSimilarity(BasicModel basicModel, AfterEffect afterEffect, Normalization normalization)	Creates DFRSimilarity from the three components.

Method Summary

Modifier and Type	Method	Description
protected void	explain(Explanation expl, BasicStats stats, int doc, float freq, float docLen)	Subclasses should implement this method to explain the score.
AfterEffect	getAfterEffect()	Returns the first normalization
BasicModel	getBasicModel()	Returns the basic model of information content
Normalization	getNormalization()	Returns the second normalization
protected float	score(BasicStats stats, float freq, float docLen)	Scores the document doc.
String	toString()	Subclasses must override this method to return the name of the Similarity and preferably the values of parameters (if any) as well.

Methods inherited from class org.apache.lucene.search.similarities.SimilarityBase

computeNorm, computeWeight, decodeNormValue, encodeNormValue, explain, fillBasicStats, getDiscountOverlaps, log2, newStats, setDiscountOverlaps, simScorer

Methods inherited from class org.apache.lucene.search.similarities.Similarity

coord, queryNorm

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Field Details

basicModel

protected final BasicModel basicModel

The basic model for information content.

afterEffect

protected final AfterEffect afterEffect

The first normalization of the information content.

normalization

protected final Normalization normalization

The term frequency normalization.

Constructor Details

DFRSimilarity

public DFRSimilarity(BasicModel basicModel,
 AfterEffect afterEffect,
 Normalization normalization)

Creates DFRSimilarity from the three components.

Note that null values are not allowed: if you want no normalization or after-effect, instead pass Normalization.NoNormalization or AfterEffect.NoAfterEffect respectively.

Parameters:

basicModel - Basic model of information content

afterEffect - First normalization of information gain

normalization - Second (length) normalization

Method Details

score

protected float score(BasicStats stats,
 float freq,
 float docLen)

Description copied from class: SimilarityBase

Scores the document doc.

Subclasses must apply their scoring formula in this class.

Specified by:

score in class SimilarityBase

Parameters:

stats - the corpus level statistics.

freq - the term frequency.

docLen - the document length.

Returns:

the score.

explain

protected void explain(Explanation expl,
 BasicStats stats,
 int doc,
 float freq,
 float docLen)

Description copied from class: SimilarityBase

Subclasses should implement this method to explain the score. expl already contains the score, the name of the class and the doc id, as well as the term frequency and its explanation; subclasses can add additional clauses to explain details of their scoring formulae.

The default implementation does nothing.

Overrides:

explain in class SimilarityBase

Parameters:

expl - the explanation to extend with details.

stats - the corpus level statistics.

doc - the document id.

freq - the term frequency.

docLen - the document length.

toString

public String toString()

Description copied from class: SimilarityBase

Subclasses must override this method to return the name of the Similarity and preferably the values of parameters (if any) as well.

Specified by:

toString in class SimilarityBase

getBasicModel

public BasicModel getBasicModel()

Returns the basic model of information content

getAfterEffect

public AfterEffect getAfterEffect()

Returns the first normalization

getNormalization

public Normalization getNormalization()

Returns the second normalization