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Experiments in the Retrieval of Unsegmented Japanese Text at the NTCIR-2 Workshop

Our work with the Hopkins Automated Information Retriever for Combing Unstructured Text (HAIRCUT) system has made use of overlapping character n-grams in the indexing and retrieval of text. In previous experiments with Western European languages we have shown that longer length n-grams (e.g., n=6) are capable of providing an effective form of alinguistic term normalization. We have wanted to investigate whether these methods could be adapted to processing unsegmented languages such as Japanese. To that end we participated in the Japanese and English portion of the NTCIR-2 evaluation. This paper describes results in monolingual Japanese and English retrieval and in cross-language retrieval using each language as a source language for the other. We found that 6-grams performed comparably with English words and that 2-grams and 3-grams perform equally well in Japanese text. A combination of runs using each tokenization method resulted in only a marginal improvement over runs using a single approach. These two trends were consistent regardless of query length or source language.