Modification of Vocabulary-based Re-ranking for Geographic and Temporal Searching at NTCIR GeoTime Task

This paper reports on experiments in the NTCIR-9 GeoTime task performed by a research group at the School of Library and Information Science in Keio University (KOLIS), which tried to explore techniques for searching a Japanese document collection for requests on geographic and temporal information. A special component of re-ranking for enhancing performance of geographic and temporal searches was added to the KOLIS system, in which standard Okapi BM25 and probabilistic pseudo-relevance feedback (PRF) were implemented. That is, at the first stage, a list of documents relevant to a given topic was specified by standard IR techniques, and at the second stage, the list was re-ranked after scores of documents which included geographic and temporal terms were increased. In particular, each score of documents including a syntactic pattern “geographic or temporal term + ” was augmented for improving search performance where “ ” is a functional word meaning “at” or “in”. In the old version of reranking used in the former NTCIR-8 workshop, only frequency of occurrence of geographic and temporal terms was taken into consideration. In this experiment of Japanese monolingual (JA-JA) retrieval and English to Japanese bilingual (EN-JA) retrieval, the search runs using jointly the re-ranking based on the syntactic pattern and PRF showed the highest performance. However, the ranking based on the syntactic pattern could not bring explicit improvement in comparison with the ranking technique at NTCIR-8.