Class LMJelinekMercerSimilarity
- java.lang.Object
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- org.apache.lucene.search.similarities.Similarity
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- org.apache.lucene.search.similarities.SimilarityBase
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- org.apache.lucene.search.similarities.LMSimilarity
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- org.apache.lucene.search.similarities.LMJelinekMercerSimilarity
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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 and0.7
for long queries.Values should be between 0 (exclusive) and 1 (inclusive). Values near zero act score more like a conjunction (coordinate level matching), whereas values near 1 behave the opposite (more like pure disjunction).
- WARNING: This API is experimental and might change in incompatible ways in the next release.
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Nested Class Summary
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Nested classes/interfaces inherited from class org.apache.lucene.search.similarities.LMSimilarity
LMSimilarity.CollectionModel, LMSimilarity.DefaultCollectionModel, LMSimilarity.LMStats
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Nested classes/interfaces inherited from class org.apache.lucene.search.similarities.Similarity
Similarity.SimScorer
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Field Summary
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Fields inherited from class org.apache.lucene.search.similarities.LMSimilarity
collectionModel
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Fields inherited from class org.apache.lucene.search.similarities.SimilarityBase
discountOverlaps
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Constructor Summary
Constructors Constructor Description LMJelinekMercerSimilarity(float lambda)
Instantiates with the specified λ parameter.LMJelinekMercerSimilarity(LMSimilarity.CollectionModel collectionModel, float lambda)
Instantiates with the specified collectionModel and λ parameter.
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Method Summary
All Methods Instance Methods Concrete Methods Modifier and Type Method Description protected void
explain(List<Explanation> subs, BasicStats stats, double freq, double docLen)
Subclasses should implement this method to explain the score.protected Explanation
explain(BasicStats stats, Explanation freq, double docLen)
Explains the score.float
getLambda()
Returns the λ parameter.String
getName()
Returns the name of the LM method.protected double
score(BasicStats stats, double freq, double docLen)
Scores the documentdoc
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Methods inherited from class org.apache.lucene.search.similarities.LMSimilarity
fillBasicStats, newStats, toString
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Methods inherited from class org.apache.lucene.search.similarities.SimilarityBase
computeNorm, getDiscountOverlaps, log2, scorer, setDiscountOverlaps
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Constructor Detail
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LMJelinekMercerSimilarity
public LMJelinekMercerSimilarity(LMSimilarity.CollectionModel collectionModel, float lambda)
Instantiates with the specified collectionModel and λ parameter.
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LMJelinekMercerSimilarity
public LMJelinekMercerSimilarity(float lambda)
Instantiates with the specified λ parameter.
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Method Detail
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score
protected double score(BasicStats stats, double freq, double docLen)
Description copied from class:SimilarityBase
Scores the documentdoc
.Subclasses must apply their scoring formula in this class.
- Specified by:
score
in classSimilarityBase
- Parameters:
stats
- the corpus level statistics.freq
- the term frequency.docLen
- the document length.- Returns:
- the score.
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explain
protected void explain(List<Explanation> subs, BasicStats stats, double freq, double 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 classLMSimilarity
- Parameters:
subs
- the list of details of the explanation to extendstats
- the corpus level statistics.freq
- the term frequency.docLen
- the document length.
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explain
protected Explanation explain(BasicStats stats, Explanation freq, double docLen)
Description copied from class:SimilarityBase
Explains the score. The implementation here provides a basic explanation in the format score(name-of-similarity, doc=doc-id, freq=term-frequency), computed from:, and attaches the score (computed via theSimilarityBase.score(BasicStats, double, double)
method) and the explanation for the term frequency. Subclasses content with this format may add additional details inSimilarityBase.explain(List, BasicStats, double, double)
.- Overrides:
explain
in classSimilarityBase
- Parameters:
stats
- the corpus level statistics.freq
- the term frequency and its explanation.docLen
- the document length.- Returns:
- the explanation.
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getLambda
public float getLambda()
Returns the λ parameter.
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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 classLMSimilarity
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