Affiliation: | aMarshall Wace LLP, 1/11 John Adam Street, London WC2N 6HT, UK bDepartment of Computer Science, Brown University, 115 Waterman Street, Box 1910, Providence, RI 02912, USA cDepartment of Statistics, Stanford University, Stanford, CA 94305, USA dDepartment of IR Systems, Swedish Defence Research Agency, SE-164 90 Stockholm, Sweden |