LETTER Communicated by Krzysztof Dembczynski´ Reduction from Cost-Sensitive Ordinal Ranking to Weighted Binary Classification Hsuan-Tien Lin
[email protected] Department of Computer Science, National Taiwan University, Taipei 106, Taiwan Ling Li
[email protected] Department of Computer Science, California Institute of Technology, Pasadena, CA 91125, U.S.A. We present a reduction framework from ordinal ranking to binary clas- sification. The framework consists of three steps: extracting extended examples from the original examples, learning a binary classifier on the extended examples with any binary classification algorithm, and con- structing a ranker from the binary classifier. Based on the framework, we show that a weighted 0/1 loss of the binary classifier upper-bounds the mislabeling cost of the ranker, both error-wise and regret-wise. Our framework allows not only the design of good ordinal ranking algo- rithms based on well-tuned binary classification approaches, but also the derivation of new generalization bounds for ordinal ranking from known bounds for binary classification. In addition, our framework uni- fies many existing ordinal ranking algorithms, such as perceptron rank- ing and support vector ordinal regression. When compared empirically on benchmark data sets, some of our newly designed algorithms enjoy ad- vantages in terms of both training speed and generalization performance over existing algorithms. In addition, the newly designed algorithms lead to better cost-sensitive ordinal ranking performance, as well as improved listwise ranking performance. 1 Introduction We work on a supervised learning problem, ordinal ranking, which is also referred to as ordinal regression (Chu & Keerthi, 2007) or ordinal clas- sification (Frank & Hall, 2001).