Efficient methods for robust classification under uncertainty in kernel matrices

  • Authors:
  • Aharon Ben-Tal;Sahely Bhadra;Chiranjib Bhattacharyya;Arkadi Nemirovski

  • Affiliations:
  • William Davidson Faculty of Industrial Engineering and Management, Technion - Israel Institute of Technology, Technion City, Haifa, Israel;Department of Computer Science and Automation, Indian Institute of Science, Bangalore, Karnataka, India;Department of Computer Science and Automation, Indian Institute of Science, Bangalore, Karnataka, India;H. Milton Stewart School of Industrial and Systems Engineering, Georgia Institute of Technology, Georgia, Atlanta, Georgia

  • Venue:
  • The Journal of Machine Learning Research
  • Year:
  • 2012

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Abstract

In this paper we study the problem of designing SVM classifiers when the kernel matrix, K, is affected by uncertainty. Specifically K is modeled as a positive affine combination of given positive semi definite kernels, with the coefficients ranging in a norm-bounded uncertainty set. We treat the problem using the Robust Optimization methodology. This reduces the uncertain SVM problem into a deterministic conic quadratic problem which can be solved in principle by a polynomial time Interior Point (IP) algorithm. However, for large-scale classification problems, IP methods become intractable and one has to resort to first-order gradient type methods. The strategy we use here is to reformulate the robust counterpart of the uncertain SVM problem as a saddle point problem and employ a special gradient scheme which works directly on the convex-concave saddle function. The algorithm is a simplified version of a general scheme due to Juditski and Nemirovski (2011). It achieves an O(1/T2) reduction of the initial error after T iterations. A comprehensive empirical study on both synthetic data and real-world protein structure data sets show that the proposed formulations achieve the desired robustness, and the saddle point based algorithm outperforms the IP method significantly.