org.apache.lucene.search.similarities
Class LMJelinekMercerSimilarity

java.lang.Object
  extended by org.apache.lucene.search.similarities.Similarity
      extended by org.apache.lucene.search.similarities.SimilarityBase
          extended by org.apache.lucene.search.similarities.LMSimilarity
              extended by org.apache.lucene.search.similarities.LMJelinekMercerSimilarity

public class LMJelinekMercerSimilarity
extends LMSimilarity

Language model based on the Jelinek-Mercer smoothing method. From Chengxiang Zhai and John Lafferty. 2001. A study of smoothing methods for language models applied to Ad Hoc information retrieval. In Proceedings of the 24th annual international ACM SIGIR conference on Research and development in information retrieval (SIGIR '01). ACM, New York, NY, USA, 334-342.

The model has a single parameter, λ. According to said paper, the optimal value depends on both the collection and the query. The optimal value is around 0.1 for title queries and 0.7 for long queries.

WARNING: This API is experimental and might change in incompatible ways in the next release.

Nested Class Summary
 
Nested classes/interfaces inherited from class org.apache.lucene.search.similarities.LMSimilarity
LMSimilarity.CollectionModel, LMSimilarity.DefaultCollectionModel, LMSimilarity.LMStats
 
Nested classes/interfaces inherited from class org.apache.lucene.search.similarities.Similarity
Similarity.ExactSimScorer, Similarity.SimWeight, Similarity.SloppySimScorer
 
Field Summary
 
Fields inherited from class org.apache.lucene.search.similarities.LMSimilarity
collectionModel
 
Fields inherited from class org.apache.lucene.search.similarities.SimilarityBase
discountOverlaps
 
Constructor Summary
LMJelinekMercerSimilarity(float lambda)
          Instantiates with the specified λ parameter.
LMJelinekMercerSimilarity(LMSimilarity.CollectionModel collectionModel, float lambda)
          Instantiates with the specified collectionModel and λ parameter.
 
Method Summary
protected  void explain(Explanation expl, BasicStats stats, int doc, float freq, float docLen)
          Subclasses should implement this method to explain the score.
 float getLambda()
          Returns the λ parameter.
 String getName()
          Returns the name of the LM method.
protected  float score(BasicStats stats, float freq, float docLen)
          Scores the document doc.
 
Methods inherited from class org.apache.lucene.search.similarities.LMSimilarity
fillBasicStats, newStats, toString
 
Methods inherited from class org.apache.lucene.search.similarities.SimilarityBase
computeNorm, computeWeight, decodeNormValue, encodeNormValue, exactSimScorer, explain, getDiscountOverlaps, log2, setDiscountOverlaps, sloppySimScorer
 
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
 

Constructor Detail

LMJelinekMercerSimilarity

public LMJelinekMercerSimilarity(LMSimilarity.CollectionModel collectionModel,
                                 float lambda)
Instantiates with the specified collectionModel and λ parameter.


LMJelinekMercerSimilarity

public LMJelinekMercerSimilarity(float lambda)
Instantiates with the specified λ parameter.

Method Detail

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 LMSimilarity
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.

getLambda

public float getLambda()
Returns the λ parameter.


getName

public String getName()
Description copied from class: LMSimilarity
Returns the name of the LM method. The values of the parameters should be included as well.

Used in LMSimilarity.toString()

.

Specified by:
getName in class LMSimilarity


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