DirichletLM (Terrier Information Retrieval Platform 5.5 API)

java.lang.Object
- org.terrier.matching.models.WeightingModel
- - org.terrier.matching.models.DirichletLM

All Implemented Interfaces:

java.io.Serializable, java.lang.Cloneable, Model
```
public class DirichletLM
extends WeightingModel
```
Bayesian smoothing with Dirichlet Prior. This has one parameter, mu > 0. "The optimal value of mu also tends to be larger for long queries than for title queries. The optimal ... seems to vary from collection to collection, though in most cases, it is around 2,000. The tail of the curves is generally flat." This class sets mu to 2500 by default. As a default, this gives higher performance than BM25 (b=0.75) on TREC Terabyte track 2004.
The retrieval performance of this weighting model has been empirically verified to be similar to that reported below. This model is formulated such that all scores are > 0.
A Study of Smoothing Methods for Language Models Applied to Information Retrieval. Zhai & Lafferty, ACM Transactions on Information Systems, Vol. 22, No. 2, April 2004, Pages 179--214.

Since:

3.0

Author:

Craig Macdonald

See Also:

Serialized Form

Field Summary
- Fields inherited from class org.terrier.matching.models.WeightingModel
  averageDocumentLength, c, cs, documentFrequency, es, i, keyFrequency, numberOfDocuments, numberOfPointers, numberOfPostings, numberOfTokens, numberOfUniqueTerms, rq, termFrequency

Constructor Summary

Constructors
Constructor Description

DirichletLM()
Constructs an instance of DirichletLM

Method Summary

All Methods Instance Methods Concrete Methods
Modifier and Type	Method	Description
`java.lang.String`	`getInfo()`	Returns the name of the model.
`double`	`score(double tf, double docLength)`	This method provides the contract for implementing weighting models.

Methods inherited from class org.terrier.matching.models.WeightingModel
clone, getOverflowed, getParameter, prepare, score, setCollectionStatistics, setEntryStatistics, setKeyFrequency, setParameter, setRequest

Methods inherited from class java.lang.Object
equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

- Constructor Detail
  - DirichletLM
```
public DirichletLM()
```
    Constructs an instance of DirichletLM
- Method Detail
  - score
```
public double score(double tf,
                    double docLength)
```
    Description copied from class: WeightingModel
    
    This method provides the contract for implementing weighting models.
    
    Specified by:
    
    score in class WeightingModel
    
    Parameters:
    
    tf - The term frequency in the document
    
    docLength - the document's length
    
    Returns:
    
    the score assigned to a document with the given tf and docLength, and other preset parameters
  - getInfo
```
public java.lang.String getInfo()
```
    Description copied from class: WeightingModel
    
    Returns the name of the model.
    
    Specified by:
    
    getInfo in interface Model
    
    Specified by:
    
    getInfo in class WeightingModel
    
    Returns:
    
    java.lang.String