Index ordering by query-independent measures

Conventional approaches to information retrieval search through all applicable entries in an inverted file for a particular collection in order to find those documents with the highest scores. For particularly large collections this may be extremely time consuming. A solution to this problem is to only search a limited amount of the collection at query-time, in order to speed up the retrieval process. In doing this we can also limit the loss in retrieval efficacy (in terms of accuracy of results). The way we achieve this is to firstly identify the most “important” documents within the collection, and sort documents within inverted file lists in order of this “importance”. In this way we limit the amount of information to be searched at query time by eliminating documents of lesser importance, which not only makes the search more efficient, but also limits loss in retrieval accuracy. Our experiments, carried out on the TREC Terabyte collection, report significant savings, in terms of number of postings examined, without significant loss of effectiveness when based on several measures of importance used in isolation, and in combination. Our results point to several ways in which the computation cost of searching large collections of documents can be significantly reduced

Enhanced information retrieval by exploiting recommender techniques in cluster-based link analysis

Inspired by the use of PageRank algorithms in document ranking, we develop and evaluate a cluster-based PageRank algorithm to re-rank information retrieval (IR) output with the objective of improving ad hoc search effectiveness. Unlike existing work, our methods exploit recommender techniques to extract the correlation between documents and apply detected correlations in a cluster-based PageRank algorithm to compute the importance of each document in a dataset. In this study two popular recommender techniques are examined in four proposed PageRank models to investigate the effectiveness of our approach. Comparison of our methods with strong baselines demonstrates the solid performance of our approach. Experimental results are reported on an extended version of the FIRE 2011 personal information retrieval (PIR) data collection which includes topically related queries with click-through data and relevance assessment data collected from the query creators. The search logs of the query creators are categorized based on their different topical interests. The experimental results show the significant improvement of our approach compared to results using standard IR and cluster-based PageRank methods

Index ordering by query-independent measures

There is an ever-increasing amount of data that is being produced from various data sources — this data must then be organised effectively if we hope to search though it. Traditional information retrieval approaches search through all available data in a particular collection in order to find the most suitable results, however, for particularly large collections this may be extremely time consuming. Our purposed solution to this problem is to only search a limited amount of the collection at query-time, in order to speed this retrieval process up. Although, in doing this we aim to limit the loss in retrieval efficacy (in terms of accuracy of results). The way we aim to do this is to firstly identify the most “important” documents within the collection, and then sort the documents within the collection in order of their "importance” in the collection. In this way we can choose to limit the amount of information to search through, by eliminating the documents of lesser importance, which should not only make the search more efficient, but should also limit any loss in retrieval accuracy. In this thesis we investigate various different query-independent methods that may indicate the importance of a document in a collection. The more accurate the measure is at determining an important document, the more effectively we can eliminate documents from the retrieval process - improving the query-throughput of the system, as well as providing a high level of accuracy in the returned results. The effectiveness of these approaches are evaluated using the datasets provided by the terabyte track at the Text REtreival Conference (TREC)

Learning to select for information retrieval

The effective ranking of documents in search engines is based on various document features, such as the frequency of the query terms in each document, the length, or the authoritativeness of each document. In order to obtain a better retrieval performance, instead of using a single or a few features, there is a growing trend to create a ranking function by applying a learning to rank technique on a large set of features. Learning to rank techniques aim to generate an effective document ranking function by combining a large number of document features. Different ranking functions can be generated by using different learning to rank techniques or on different document feature sets. While the generated ranking function may be uniformly applied to all queries, several studies have shown that different ranking functions favour different queries, and that the retrieval performance can be significantly enhanced if an appropriate ranking function is selected for each individual query. This thesis proposes Learning to Select (LTS), a novel framework that selectively applies an appropriate ranking function on a per-query basis, regardless of the given query's type and the number of candidate ranking functions. In the learning to select framework, the effectiveness of a ranking function for an unseen query is estimated from the available neighbouring training queries. The proposed framework employs a classification technique (e.g. k-nearest neighbour) to identify neighbouring training queries for an unseen query by using a query feature. In particular, a divergence measure (e.g. Jensen-Shannon), which determines the extent to which a document ranking function alters the scores of an initial ranking of documents for a given query, is proposed for use as a query feature. The ranking function which performs the best on the identified training query set is then chosen for the unseen query. The proposed framework is thoroughly evaluated on two different TREC retrieval tasks (namely, Web search and adhoc search tasks) and on two large standard LETOR feature sets, which contain as many as 64 document features, deriving conclusions concerning the key components of LTS, namely the query feature and the identification of neighbouring queries components. Two different types of experiments are conducted. The first one is to select an appropriate ranking function from a number of candidate ranking functions. The second one is to select multiple appropriate document features from a number of candidate document features, for building a ranking function. Experimental results show that our proposed LTS framework is effective in both selecting an appropriate ranking function and selecting multiple appropriate document features, on a per-query basis. In addition, the retrieval performance is further enhanced when increasing the number of candidates, suggesting the robustness of the learning to select framework. This thesis also demonstrates how the LTS framework can be deployed to other search applications. These applications include the selective integration of a query independent feature into a document weighting scheme (e.g. BM25), the selective estimation of the relative importance of different query aspects in a search diversification task (the goal of the task is to retrieve a ranked list of documents that provides a maximum coverage for a given query, while avoiding excessive redundancy), and the selective application of an appropriate resource for expanding and enriching a given query for document search within an enterprise. The effectiveness of the LTS framework is observed across these search applications, and on different collections, including a large scale Web collection that contains over 50 million documents. This suggests the generality of the proposed learning to select framework. The main contributions of this thesis are the introduction of the LTS framework and the proposed use of divergence measures as query features for identifying similar queries. In addition, this thesis draws insights from a large set of experiments, involving four different standard collections, four different search tasks and large document feature sets. This illustrates the effectiveness, robustness and generality of the LTS framework in tackling various retrieval applications

Information search in web archives

Tese de doutoramento, Informática (Engenharia Informática), Universidade de Lisboa, Faculdade de Ciências, 2014Web archives preserve information that was published on the web or digitized from printed publications. Many of that information is unique and historically valuable. However, users do not have dedicated tools to find the desired information, which hampers the usefulness of web archives. This dissertation investigates solutions towards the advance of web archive information retrieval (WAIR) and contributes to the increase of knowledge about its technology and users. The thesis underlying this work is that the search results can be improved by exploiting temporal information intrinsic to web archives. This temporal information was leveraged from two different angles. First, the long-term persistence of web documents was analyzed and modeled to better estimate their relevance to a query. Second, a temporal-dependent ranking framework that learns and combines ranking models specific for each period was devised. This approach contrasts with a typical single-model approach that ignores the variance of web characteristics over time. The proposed approach was empirically validated through various controlled experiments that demonstrated their superiority over the state-of-the-art in WAIR.Os arquivos da web preservam informação que foi publicada na web ou digitalizada de publicações impressas. Muita dessa informação é única e historicamente valiosa. Contudo, os utilizadores não dispõem de ferramentas dedicadas para encontrar a informação desejada, o que limita a utilidade dos arquivos da web. Esta dissertação investiga soluções para o avanço da recuperação de informação em arquivos da web (WAIR) e contribui para o aumento de conhecimento acerca da sua tecnologia e dos seus utilizadores. A tese subjacente a este trabalho é a de que os resultados de pesquisa podem ser melhorados através da exploração de informação temporal intrínseca aos arquivos da web. Esta informação temporal foi explorada de dois ângulos diferentes. Primeiro, a longa persistência dos documentos web foi analisada e modelada para melhor estimar a relevância destes em função da pesquisa. Segundo, foi concebido um enquadramento (framework) para ordenação de resultados dependente do tempo, que aprende e combina modelos específicos para cada período. Esta abordagem contrasta com a abordagem de um modelo único que ignora a variação das características da web ao longo do tempo. A abordagem proposta foi validada empiricamente através de várias experiências controladas que demonstraram a sua superioridade em relação ao estado da arte em WAIR

Using community trained recommender models for enhanced information retrieval

Research in Information Retrieval (IR) seeks to develop methods which better assist users in finding information which is relevant to their current information needs. Personalization is a significant focus of research for the development of next generation of IR systems. Commercial search engines are exploring methods to incorporate models of the user’s interests to facilitate personalization in IR to improve retrieval effectiveness. However, in some situations there may be no opportunity to learn about the interests of a specific user on a certain topic. This is a significant challenge for IR researchers attempting to improve search effectiveness by exploiting user search behaviour. We propose a solution to this problem based on recommender systems (RSs) in a novel IR model which combines a recommender model with traditional IR methods to improve retrieval results for search tasks, where the IR system has no opportunity to acquire prior information about the user’s knowledge of a domain for which they have not previously entered a query. We use search behaviour data from other previous users to build topic category models based on topic interests. When a user enters a query on a topic which is new to this user, but related to a topical search category, the appropriate topic category model is selected and used to predict a ranking which this user may find interesting based on previous search behaviour. The recommender outputs are used in combination with the output of a standard IR system to produce the overall output to the user. In this thesis, the IR and recommender components of this integrated model are investigated