... ‘tropical storms’, retrieving a documentonly containing tropical can be meaningless, but adocument about storm can be meaningful. Based on this observation, we only incorporate documentfrequency ... arepotentially useful for not only discriminating aphrase itself but characterizing its constituents. Inthis section, we report only the ones that havemade positive contributions to the overall retrievalperformance. ... evidence that ithas to be considered in phrase weighting. It seems to be a natural conclusion, because the importanceof the modifier word in retrieval is subordinate to the relation to its headword,...