Ping Li, Qiang Wu, Christopher Burges
We cast the ranking problem as (1) multiple classiﬁcation (“Mc”) (2) multiple or- dinal classiﬁcation, which lead to computationally tractable learning algorithms for relevance ranking in Web search. We consider the DCG criterion (discounted cumulative gain), a standard quality measure in information retrieval. Our ap- proach is motivated by the fact that perfect classiﬁcations result in perfect DCG scores and the DCG errors are bounded by classiﬁcation errors. We propose us- ing the Expected Relevance to convert class probabilities into ranking scores. The class probabilities are learned using a gradient boosting tree algorithm. Evalua- tions on large-scale datasets show that our approach can improve LambdaRank  and the regressions-based ranker , in terms of the (normalized) DCG scores. An efﬁcient implementation of the boosting tree algorithm is also presented.