Class LMJelinekMercerSimilarity
java.lang.Object
org.apache.lucene.search.similarities.Similarity
org.apache.lucene.search.similarities.SimilarityBase
org.apache.lucene.search.similarities.LMSimilarity
org.apache.lucene.search.similarities.LMJelinekMercerSimilarity
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.
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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.SimScorer, Similarity.SimWeight
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Field Summary
Fields inherited from class org.apache.lucene.search.similarities.LMSimilarity
collectionModel
Fields inherited from class org.apache.lucene.search.similarities.SimilarityBase
discountOverlaps
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Constructor Summary
ConstructorsConstructorDescriptionLMJelinekMercerSimilarity
(float lambda) Instantiates with the specified λ parameter.LMJelinekMercerSimilarity
(LMSimilarity.CollectionModel collectionModel, float lambda) Instantiates with the specified collectionModel and λ parameter. -
Method Summary
Modifier and TypeMethodDescriptionprotected void
explain
(Explanation expl, BasicStats stats, int doc, float freq, float docLen) Subclasses should implement this method to explain the score.float
Returns the λ parameter.getName()
Returns the name of the LM method.protected float
score
(BasicStats stats, float freq, float docLen) Scores the documentdoc
.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, explain, getDiscountOverlaps, log2, setDiscountOverlaps, simScorer
Methods inherited from class org.apache.lucene.search.similarities.Similarity
coord, queryNorm
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Constructor Details
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LMJelinekMercerSimilarity
Instantiates with the specified collectionModel and λ parameter. -
LMJelinekMercerSimilarity
public LMJelinekMercerSimilarity(float lambda) Instantiates with the specified λ parameter.
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Method Details
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score
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
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:
expl
- the explanation to extend with details.stats
- the corpus level statistics.doc
- the document id.freq
- the term frequency.docLen
- the document length.
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getLambda
public float getLambda()Returns the λ parameter. -
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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