Krisztian Balog Toine Bogers Leif Azzopardi Maarten de Rijke Antal van den Bosch Broad expertise retrieval in sparse data environments. 551-558 2007 conf/sigir/2007 SIGIR http://doi.acm.org/10.1145/1277741.1277836 db/conf/sigir/sigir2007.html#BalogBARB07