Multi-lingual Indexing Support for CLIR using Language Modeling

An indexing model is the heart of an Information Retrieval (IR) system. Data structures such as term based inverted indices have proved to be very effective for IR using vector space retrieval models. However, when functional aspects of such models were tested, it was soon felt that better relevance models were required to more accurately compute the relevance of a document towards a query. It was shown that language modeling approaches [1] in monolingual IR tasks improve the quality of search results in comparison with TFIDF [2] algorithm. The disadvantage of language modeling approaches when used in monolingual IR task as suggested in [1] is that they would require both the inverted index (term-todocument) and the forward index (document-to-term) to be able to compute the rank of document for a given query. This calls for an additional space and computation overhead when compared to inverted index models. Such a cost may be acceptable if the quality of search results are significantly improved. In a Cross-lingual IR (CLIR) task, we have previously shown in [3] that using a bilingual dictionary along with term co-occurrence statistics and language modeling approach helps improve the functional IR performance. However, no studies exist on the performance overhead in a CLIR task due to language modeling. In this paper we present an augmented index model which can be used for fast retrieval while having the benefits of language modeling in a CLIR task. The model is capable of retrieval and ranking with or without query expansion techniques using term collocation statistics of the indexed corpus. Finally we conduct performance related experiments on our indexing model to determine the cost overheads on space and time.