Put three and three together: Triangle-driven community detection

Community detection has arisen as one of the most relevant topics in the field of graph data mining due to its applications in many fields such as biology, social networks, or network traffic analysis. Although the existing metrics used to quantify the quality of a community work well in general, under some circumstances, they fail at correctly capturing such notion. The main reason is that these metrics consider the internal community edges as a set, but ignore how these actually connect the vertices of the community. We propose the Weighted Community Clustering (WCC), which is a new community metric that takes the triangle instead of the edge as the minimal structural motif indicating the presence of a strong relation in a graph. We theoretically analyse WCC in depth and formally prove, by means of a set of properties, that the maximization of WCC guarantees communities with cohesion and structure. In addition, we propose Scalable Community Detection (SCD), a community detection algorithm based on WCC, which is designed to be fast and scalable on SMP machines, showing experimentally that WCC correctly captures the concept of community in social networks using real datasets. Finally, using ground-truth data, we show that SCD provides better quality than the best disjoint community detection algorithms of the state of the art while performing faster.Peer ReviewedPostprint (author's final draft

Massive query expansion by exploiting graph knowledge bases for image retrieval

Annotation-based techniques for image retrieval suffer from sparse and short image textual descriptions. Moreover, users are often not able to describe their needs with the most appropriate keywords. This situation is a breeding ground for a vocabulary mismatch problem resulting in poor results in terms of retrieval precision. In this paper, we propose a query expansion technique for queries expressed as keywords and short natural language descriptions. We present a new massive query expansion strategy that enriches queries using a graph knowledge base by identifying the query concepts, and adding relevant synonyms and semantically related terms. We propose a topological graph enrichment technique that analyzes the network of relations among the concepts, and suggests semantically related terms by path and community detection analysis of the knowledge graph. We perform our expansions by using two versions of Wikipedia as knowledge base achieving improvements of the system's precision up to more than 27% Copyright 2014 ACM.Peer Reviewe

Massive query expansion by exploiting graph knowledge bases for image retrieval

Annotation-based techniques for image retrieval suffer from sparse and short image textual descriptions. Moreover, users are often not able to describe their needs with the most appropriate keywords. This situation is a breeding ground for a vocabulary mismatch problem resulting in poor results in terms of retrieval precision. In this paper, we propose a query expansion technique for queries expressed as keywords and short natural language descriptions. We present a new massive query expansion strategy that enriches queries using a graph knowledge base by identifying the query concepts, and adding relevant synonyms and semantically related terms. We propose a topological graph enrichment technique that analyzes the network of relations among the concepts, and suggests semantically related terms by path and community detection analysis of the knowledge graph. We perform our expansions by using two versions of Wikipedia as knowledge base achieving improvements of the system's precision up to more than 27% Copyright 2014 ACM.Peer Reviewe

Query expansion by relying on the structure of knowledge bases

Query expansion techniques aim at improving the results achieved by a user's query by means of introducing new expansion terms, called expansion features. Expansion features introduce new concepts that are semantically related with the concepts in the user's query and that allow retrieving documents that otherwise would be not. Thus, the challenge is to select those expansion features that are capable of improving the results the most. A bad choice of expansion features may be counterproductive. In this thesis, we use an external source of information, a Knowledge Base (KB), as source expansion features. A knowledge base consists of a set of entries, each of which represent a concept and has, at least, a name, which can be used as expansion feature. The techniques framed in this family have become more popular due to the increase of available data, as, for example, Wikipedia. Particularly, we focus on exploiting those KB whose entries are linked to each other, conforming a graph of entries. To the best of our knowledge, most of the techniques framed on the KB family rely on some kind of text analysis, such as explicit semantic analysis, or are based on other existing query expansion techniques such as pseudo relevance feedback. However, the underlying net-work structure of KBs has been barely exploited. In this thesis, we show that the structure can be used to identify reliable expansion feature for the query expansion process. Thus, we design a novel expansion technique, Structural Query Expansion (SQE). For SQE to benefit from the particular structures of KBs, we propose a methodology to identify the structural characteristics that, given a query, allow identifying those nodes in the KB that are good candidates to be used as source of expansion features, called from now on expansion nodes. The methodology consists in building a ground truth that connects each query from a query set with those nodes of the KB that when used to extract the expansion features allow achieving the best results in terms of precision, we call the set of those nodes, expansion query graph. Then, we compare the expansion query graph of each query to find shared characteristics. SQE materializes the revealed characteristics into a set of structural motifs. In the particular case of Wikipedia, we have found two motifs called triangular and square. In the former, the query node and the expansion node are doubly linked and the expansion node belongs to, at least, the same categories as the query node. In the latter, the query node and the expansion node also are doubly linked and their categories are connected somehow. These motifs are used to, given a query and its query nodes, identify all the expansion nodes which are used as source of expansion features. Notice that we have designed this technique to be orthogonal to others because is fully decoupled from the search process and does not depend on the particular collection of documents. We have tested our techniques with three different datasets to avoid any kind of overfitting. The results are shown to be consistent among the three of them. Also, the results which are validated with statistical significance tests, show that SQE is capable to achieve up to 150% improvement in the precision. Finally, we show the performance of our technique which runs in sub-second times (358.23ms at maximum) which makes it feasible for a real query expansion system. This is especially relevant because, to the best of our knowledge, the performance is an aspect that is being ignored in most of the works and, thus, it is difficult to know whether they can be include in real systems or not.Les tècniques d'expansió de consultes tenen com a objecte millorar els resultats obtinguts per la consulta d'un usuari a partir de la introducció de termes d'expansió, anomenat característiques d'expansió. Les característiques d'expansió introdueixen nous conceptes que estan relacionats semànticament amb els conceptes de la consulta de l'usuari i que permeten obtenir documents que d'altra manera no es podrien obtenir. Per tant, el repte és seleccionar les característiques d'expansió que són capaces de millorar al màxim els resultats, doncs una mala elecció pot ser contra-productiva. En aquesta tesis, utilitzem una font externa d'informació, una Base de Coneixement (KB), com a font de característiques d'expansió. Una KB és un conjunt d'entrades, cadascuna de les quals representa un concepte i que té, com a mínim, un nom, que és susceptible de ser usat com a característica d'expansió. Les tècniques emmarcades en aquesta família han esdevingut populars degut al creixement de la informació disponible, per exemple, Wikipedia. Particularment, nosaltres en centrem en utilitzar aquelles KB les entrades de les quals estan relacionades entre si, conformant d'aquesta manera, un graf d'entrades. Segons les nostres informacions, la majora de les tècniques emmarcades en aquesta família utilitzen algun tipus d'anàlisi lingüístic, o estan basades en d'altres tècniques com relevance feedback. Ara bé, la estructura subjacent de la xarxa gairebé no s'ha utilitzat. En aquesta tesis, mostrem que la estructura es pot fer servir per identificar característiques d'expansió fiables pel procés d'expansió de consultes. De fet, proposem una tècnica d'expansió novell, Structural Query Expansion (SQE), que la explota. Perquè SQE pugui beneficiar-se de les particularitats estructurals de les KBs, hem proposat també una metodologia per revelar les característiques estructurals que, donada una consulta, permeten identificar aquells nodes que són una bona font de característiques d'expansió, els anomenats, nodes d'expansió. Aquesta metodologia consisteix en construir un ground truth que relaciona una conjunt de consultes amb el seu optimal expansion query graph. L'optimal expansion query graph és el conjunt de nodes d'expansió que quan s'utilitzen com a font de característiques d'expansió, permeten obtenir els millors resultats en termes de precisió. Un cop tenim els optimal expansion query graphs, els comparem entre si per a buscar característiques compartides. SQE materialitza aquestes característiques en un conjunt de motius estructurals. En el cas de Wikipedia hem trobat 2 motius: el triangular i el quadràtic. En els dos casos el node de la consulta ha d'estar doblement lincat amb el node d'expansió. En el triangular, les categories del node d'expansió ha de pertànyer, com a mínim, a les mateixes categories que el node de la consulta, mentre que en el quadràtic tan sols cal que les categories del node de la consulta i el d'expansió estiguin relacionades. Aquest motius s'utilitzen per, donada una consulta, identificar tots els seus nodes d'expansió. Hem dissenyat aquesta tècnica com una tècnica ortogonal a d'altres ja que està desacoblada del procés de cerca i no depèn de la col·lecció de documents. Hem provar la nostra tècnica amb 3 jocs de dades diferents per a evitar qualsevol tipus d'especialització. Els resultats són consistents entre els tres. Hem validat els resultats amb testos de significança estadística obtenint millores del 150% en la precisió. Finalment, pel que fa el rendiment de la nostra proposta, mostrem que s'executa en mil·lisegons, i això la fa susceptible de ser utilitzada en sistemes d'expansió reals. Això és especialment rellevant perquè, segons les nostres informacions, aquest és un aspecte que s'ignora en la literatura i, per tant, és difícil de saber la viabilitat de les propostes que existeixen en entorns reals