This paper explores log-based query expansion ( QE ) models for Web search . Three lexicon models are proposed to bridge the lexical gap between Web documents and user queries . These models are trained on pairs of user queries and titles of clicked documents . Evaluations on a real world data set show that the lexicon models , integrated into a ranker-based QE system , not only significantly improve the document retrieval performance but also outperform two state-of-the-art log-based QE methods . Term mismatch is a fundamental problem in Web search , where queries and documents are composed using different vocabularies and language styles . Query expansion ( QE ) is an effective strategy to address the problem . It expands a query issued by a user with additional related terms , called expansion terms , so that more relevant documents can be retrieved . In this paper we explore the use of clickthrough data and translation models for QE . We select expansion terms for a query according to how likely it is that the expansion terms occur in the title of a document that is relevant to the query . Assuming that a query is parallel to the titles of documents clicked for that query ( Gao et al . 2010a ) , three lexicon models are trained on query-title pairs extracted from clickthrough data . The first is a word model that learns the translation probability between single words . The second model uses lexi-calized triplets to incorporate word dependencies for translation . The third is a bilingual topic model , which represents a query as a distribution of hidden topics and learns the translation between a query and a title term at the semantic level . We will show that the word model provides a rich set of expansion candidates while the triplet and topic models can effectively select good expansion terms , and that a ranker-based QE system which incorporates all three of these models not only significantly improves Web search result but outperforms other log-based QE methods that are stateof-the-art . There is growing interest in applying user logs to improve QE . A recent survey is due to Baeze-Yates and Ribeiro-Neto ( 2011 ) . Below , we briefly discuss two log-based QE methods that are closest to ours and are re-implemented in this study for comparison . Both systems use the same type of log data that we used to train the lexicon models . The term correlation model of Cui et al . ( 2002 ; 2003 ) is to our knowledge the first to explore querydocument relations for direct extraction of expansion terms for Web search . The method outperforms traditional QE methods that do not use log data e.g. the local analysis model of Xu and Croft ( 1996 ) . In addition , as pointed out by Cui et al . ( 2003 ) there are three important advantages that make log-based QE a promising technology to improve the performance of commercial search engines . First , unlike traditional QE methods that are based on relevance feedback , log-based QE derives expansion terms from search logs , allowing term correlations to be pre-computed offline . Compared to methods that are based on thesauri either compiled manually ( Prager et al . 2001 ) or derived au-tomatically from document collections ( Jing and Croft 1994 ) , the log-based method is superior in that it explicitly captures the correlation between query terms and document terms , and thus can bridge the lexical gap between them more effectively . Second , since search logs retrain querydocument pairs clicked by millions of users , the term correlations reflect the preference of the majority of users . Third , the term correlations evolve along with the accumulation of user logs , thus can reflect updated user interests at a specific time . However , as pointed out by Riezler et al . ( 2008 ) , Cui et al . 's correlation-based method suffers low precision of QE partly because the correlation model does not explicitly capture context information and is susceptible to noise . Riezler et al . developed a QE system by retraining a standard phrase-based statistical machine translation ( SMT ) system using query-snippet pairs extracted from clickthrough data ( Riezler et al . 2008 ; Riezler and Liu 2010 ) . The SMT-based system can produce cleaner , more relevant expansion terms because rich context information useful for filtering noisy expansions is captured by combining language model and phrase translation model in its decoder . Furthermore , in the SMT system all component models are properly smoothed using sophisticated techniques to avoid sparse data problems while the correlation model relies on pure counts of term frequencies . However , the SMT system is used as a black box in their experiments . So the relative contribution of different SMT components is not verified empirically . In this study we break this black box in order to build a better , simpler QE system . We will show that the proposed lexicon models outperform significantly the term correlation model , and that a simpler QE system that incorporates the lexicon models can beat the sophisticated , black-box SMT system . In this paper we extend the previous log-based QE methods in two directions . First , we formulate QE as the problem of translating a source language of queries into a target language of documents , represented as titles . This allows us to adapt the established techniques developed for SMT to QE . Specially , we propose three lexicon models based on terms , lexicalized triplets , and topics , respectively . These models are trained on pairs of user queries and the titles of clicked documents using EM . Second , we present a ranker-based QE system , the heart of which is a MRF-based ranker in which the lexicon models are incorporated as features . We perform experiments on the Web search task using a real world data set . Results show that the proposed system outperforms significantly other stateof-the-art QE systems . This study is part of a bigger , ongoing project , aiming to develop a real-time QE system for Web search , where simplicity is the key to the success . Thus , what we learned from this study is particularly encouraging . We demonstrate that with large amounts of clickthrough data for model training , simple lexicon models can achieve state-of-the-art QE performance , and that the MRF-based ranker provides a simple and flexible framework to incorporate a variety of features capturing different types of term dependencies in such an effective way that the Web search performance can be directly optimized .
Term mismatches between a query and documents hinder retrievals of relevant documents and black box statistical machine translation models are used to expand queries.
They propose to train lexicon query expansion models by using transaction logs that contain pairs of queries and titles of clicked documents.
The proposed query expansion model enables retrieval systems to significantly outperform models with previous expansion models while being more transparent.