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## org.apache.lucene.search.similarities
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

    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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