Berkant Barla Cambazoglu Vassilis Plachouras Ricardo A. Baeza-Yates Quantifying performance and quality gains in distributed web search engines. 411-418 2009 SIGIR http://doi.acm.org/10.1145/1571941.1572013 conf/sigir/2009 db/conf/sigir/sigir2009.html#CambazogluPB09