Falk Scholer

User performance versus precision measures for simple search tasks

Several recent studies have demonstrated that the type of improvements in information retrieval system effectiveness reported in forums such as SIGIR and TREC do not translate into a benefit for users. Two of the studies used an instance recall task, and a third used a question answering task, so perhaps it is unsurprising that the precision based measures of IR system effectiveness on one-shot query evaluation do not correlate with user performance on these tasks. In this study, we evaluate two different information retrieval tasks on TREC Web-track data: a precision-based user task, measured by the length of time that users need to find a single document that is relevant to a TREC topic; and, a simple recall-based task, represented by the total number of relevant documents that users can identify within five minutes. Users employ search engines with controlled mean average precision (MAP) of between 55% and 95%. Our results show that there is no significant relationship between system effectiveness measured by MAP and the precision-based task. A significant, but weak relationship is present for the precision at one document returned metric. A weak relationship is present between MAP and the simple recall-based task.

Compression of inverted indexes For fast query evaluation

Compression reduces both the size of indexes and the time needed to evaluate queries. In this paper, we revisit the compression of inverted lists of document postings that store the position and frequency of indexed terms, considering two approaches to improving retrieval efficiency: better implementation and better choice of integer compression schemes. First, we propose several simple optimisations to well-known integer compression schemes, and show experimentally that these lead to significant reductions in time. Second, we explore the impact of choice of compression scheme on retrieval efficiency.In experiments on large collections of data, we show two surprising results: use of simple byte-aligned codes halves the query evaluation time compared to the most compact Golomb-Rice bitwise compression schemes; and, even when an index fits entirely in memory, byte-aligned codes result in faster query evaluation than does an uncompressed index, emphasising that the cost of transferring data from memory to the CPU cache is less for an appropriately compressed index than for an uncompressed index. Moreover, byte-aligned schemes have only a modest space overhead: the most compact schemes result in indexes that are around 10% of the size of the collection, while a byte-aligned scheme is around 13%. We conclude that fast byte-aligned codes should be used to store integers in inverted lists.

Query expansion using associated queries

Hundreds of millions of users each day use web search engines to meet their information needs. Advances in web search effectiveness are therefore perhaps the most significant public outcomes of IR research. Query expansion is one such method for improving the effectiveness of ranked retrieval by adding additional terms to a query. In previous approaches to query expansion, the additional terms are selected from highly ranked documents returned from an initial retrieval run. We propose a new method of obtaining expansion terms, based on selecting terms from past user queries that are associated with documents in the collection. Our scheme is effective for query expansion for web retrieval: our results show relative improvements over unexpanded full text retrieval of 26%--29%, and 18%--20% over an optimised, conventional expansion approach.

Effective pre-retrieval query performance prediction using similarity and variability evidence

Query performance prediction aims to estimate the quality of answers that a search system will return in response to a particular query. In this paper we propose a new family of pre-retrieval predictors based on information at both the collection and document level. Pre-retrieval predictors are important because they can be calculated from information that is available at indexing time; they are therefore more efficient than predictors that incorporate information obtained from actual search results. Experimental evaluation of our approach shows that the new predictors give more consistent performance than previously proposed pre-retrieval methods across a variety of data types and search tasks.

Capturing collection size for distributed non-cooperative retrieval

Modern distributed information retrieval techniques require accurate knowledge of collection size. In non-cooperative environments, where detailed collection statistics are not available, the size of the underlying collections must be estimated. While several approaches for the estimation of collection size have been proposed, their accuracy has not been thoroughly evaluated. An empirical analysis of past estimation approaches across a variety of collections demonstrates that their prediction accuracy is low. Motivated by ecological techniques for the estimation of animal populations, we propose two new approaches for the estimation of collection size. We show that our approaches are significantly more accurate that previous methods, and are more efficient in use of resources required to perform the estimation.

Quantifying test collection quality based on the consistency of relevance judgements

Relevance assessments are a key component for test collection-based evaluation of information retrieval systems. This paper reports on a feature of such collections that is used as a form of ground truth data to allow analysis of human assessment error. A wide range of test collections are retrospectively examined to determine how accurately assessors judge the relevance of documents. Our results demonstrate a high level of inconsistency across the collections studied. The level of irregularity is shown to vary across topics, with some showing a very high level of assessment error. We investigate possible influences on the error, and demonstrate that inconsistency in judging increases with time. While the level of detail in a topic specification does not appear to influence the errors that assessors make, judgements are significantly affected by the decisions made on previously seen similar documents. Assessors also display an assessment inertia. Alternate approaches to generating relevance judgements appear to reduce errors. A further investigation of the way that retrieval systems are ranked using sets of relevance judgements produced early and late in the judgement process reveals a consistent influence measured across the majority of examined test collections. We conclude that there is a clear value in examining, even inserting, ground truth data in test collections, and propose ways to help minimise the sources of inconsistency when creating future test collections.

Including summaries in system evaluation

In batch evaluation of retrieval systems, performance is calculated based on predetermined relevance judgements applied to a list of documents returned by the system for a query. This evaluation paradigm, however, ignores the current standard operation of search systems which require the user to view summaries of documents prior to reading the documents themselves. In this paper we modify the popular IR metrics MAP and P@10 to incorporate the summary reading step of the search process, and study the effects on system rankings using TREC data. Based on a user study, we establish likely disagreements between relevance judgements of summaries and of documents, and use these values to seed simulations of summary relevance in the TREC data. Re-evaluating the runs submitted to the TREC Web Track, we find the average correlation between system rankings and the original TREC rankings is 0.8 (Kendall τ), which is lower than commonly accepted for system orderings to be considered equivalent. The system that has the highest MAP in TREC generally remains amongst the highest MAP systems when summaries are taken into account, but other systems become equivalent to the top ranked system depending on the simulated summary relevance. Given that system orderings alter when summaries are taken into account, the small amount of effort required to judge summaries in addition to documents (19 seconds vs 88 seconds on average in our data) should be undertaken when constructing test collections.

Machine transliteration survey

Machine transliteration is the process of automatically transforming the script of a word from a source language to a target language, while preserving pronunciation. The development of algorithms specifically for machine transliteration began over a decade ago based on the phonetics of source and target languages, followed by approaches using statistical and language-specific methods. In this survey, we review the key methodologies introduced in the transliteration literature. The approaches are categorized based on the resources and algorithms used, and the effectiveness is compared.

Query association surrogates for Web search

Collection sizes, query rates, and the number of users of Web search engines are increasing. Therefore, there is continued demand for innovation in providing search services that meet user information needs. In this article, we propose new techniques to add additional terms to documents with the goal of providing more accurate searches. Our techniques are based on query association, where queries are stored with documents that are highly similar statistically. We show that adding query associations to documents improves the accuracy of Web topic finding searches by up to 7%, and provides an excellent complement to existing supplement techniques for site finding. We conclude that using document surrogates derived from query association is a valuable new technique for accurate Web searching.

Users versus models: what observation tells us about effectiveness metrics

Retrieval system effectiveness can be measured in two quite different ways: by monitoring the behavior of users and gathering data about the ease and accuracy with which they accomplish certain specified information-seeking tasks; or by using numeric effectiveness metrics to score system runs in reference to a set of relevance judgments. In the second approach, the effectiveness metric is chosen in the belief that user task performance, if it were to be measured by the first approach, should be linked to the score provided by the metric. This work explores that link, by analyzing the assumptions and implications of a number of effectiveness metrics, and exploring how these relate to observable user behaviors. Data recorded as part of a user study included user self-assessment of search task difficulty; gaze position; and click activity. Our results show that user behavior is influenced by a blend of many factors, including the extent to which relevant documents are encountered, the stage of the search process, and task difficulty. These insights can be used to guide development of batch effectiveness metrics.