Filip Radlinski Andrei Z. Broder Peter Ciccolo Evgeniy Gabrilovich Vanja Josifovski Lance Riedel Optimizing relevance and revenue in ad search: a query substitution approach. 403-410 2008 SIGIR http://doi.acm.org/10.1145/1390334.1390404 conf/sigir/2008 db/conf/sigir/sigir2008.html#RadlinskiBCGJR08