Index Pruning and Result Reranking: Effects on Ad-Hoc Retrieval and Named Page Finding

We describe experiments conducted for the TREC 2006 Terabyte track. Our experiments are centered around two concepts: Static index pruning (for increased retrieval efficiency) and result reranking (for improved precision). We investigate their effect on retrieval efficiency and effectiveness, paying special attention to the difference between ad-hoc retrieval and named page finding. We show that index pruning and reranking based on relevance models can be beneficial in an ad-hoc retrieval setting, but have a disastrous repercussion on the effectiveness of named page finding. Result reranking based on anchor text, on the other hand, is very useful for named page finding, but should not be used for ad-hoc retrieval. This dichotomy poses a problem for search engines, as there is no easy way for a search engine to decide whether a given query represents an ad-hoc retrieval task, with the purpose to satisfy an abstract information need, or a named page finding task, targeting a specific document.