Gianluca Demartini Paul-Alexandru Chirita Ingo Brunkhorst Wolfgang Nejdl Ranking Categories for Web Search. 564-569 2008 ECIR http://dx.doi.org/10.1007/978-3-540-78646-7_56 conf/ecir/2008 db/conf/ecir/ecir2008.html#DemartiniCBN08