Giambattista Amati Claudio Carpineto Giovanni Romano Query Difficulty, Robustness, and Selective Application of Query Expansion. 127-137 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2997&spage=127 2004 conf/ecir/2004 ECIR db/conf/ecir/ecir2004.html#AmatiCR04