org.apache.lucene.search.similarities

Class LMDirichletSimilarity

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.LMDirichletSimilarity

public class LMDirichletSimilarity
extends LMSimilarity

Bayesian smoothing using Dirichlet priors. 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 formula as defined the paper assigns a negative score to documents that contain the term, but with fewer occurrences than predicted by the collection language model. The Lucene implementation returns 0 for such documents.

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

Constructor Summary

Constructors

Constructor and Description
LMDirichletSimilarity() Instantiates the similarity with the default μ value of 2000.
LMDirichletSimilarity(float mu) Instantiates the similarity with the provided μ parameter.
LMDirichletSimilarity(LMSimilarity.CollectionModel collectionModel) Instantiates the similarity with the default μ value of 2000.
LMDirichletSimilarity(LMSimilarity.CollectionModel collectionModel, float mu) Instantiates the similarity with the provided μ parameter.

Method Summary

Modifier and Type	Method and Description
float	getMu() Returns the μ parameter.
String	getName() Returns the name of the LM method.

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

toString

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

computeNorm, computeWeight, getDiscountOverlaps, log2, setDiscountOverlaps, simScorer

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

coord, queryNorm

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, wait, wait, wait

Constructor Detail

LMDirichletSimilarity

public LMDirichletSimilarity(LMSimilarity.CollectionModel collectionModel,
                             float mu)

Instantiates the similarity with the provided μ parameter.

LMDirichletSimilarity

public LMDirichletSimilarity(float mu)

Instantiates the similarity with the provided μ parameter.

LMDirichletSimilarity

public LMDirichletSimilarity(LMSimilarity.CollectionModel collectionModel)

Instantiates the similarity with the default μ value of 2000.

LMDirichletSimilarity

public LMDirichletSimilarity()

Instantiates the similarity with the default μ value of 2000.

Method Detail

getMu

public float getMu()

Returns the μ 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