Graph-of-word and TW-IDF: New approach to Ad Hoc IR

In this paper, we introduce novel document representation (graph-of-word) and retrieval model (TW-IDF) for ad hoc IR. Questioning the term independence assumption behind the traditional bag-of-word model, we propose a different representation of a document that captures the relationships between the terms using an unweighted directed graph of terms. From this graph, we extract at indexing time meaningful term weights (TW) that replace traditional term frequencies (TF) and from which we define a novel scoring function, namely TW-IDF, by analogy with TF-IDF. This approach leads to a retrieval model that consistently and significantly outperforms BM25 and in some cases its extension BM25+ on various standard TREC datasets. In particular, experiments show that counting the number of different contexts in which a term occurs inside a document is more effective and relevant to search than considering an overall concave term frequency in the context of ad hoc IR.