Paper:
Regularization Path of Cross-Validation Error Lower Bounds
Careful tuning of a regularization parameter is indispensable in many machine learning tasks because it has a significant impact on generalization performances. Nevertheless, current practice of regularization parameter tuning is more of an art than a science, e.g., it is hard to tell how many grid-points would be needed in cross-validation (CV) for obtaining a solution with sufficiently small CV error. In this paper we propose a novel framework for computing a lower bound of the CV errors as a function of the regularization parameter, which we call regularization path of CV error lower bounds. The proposed framework can be used for providing a theoretical approximation guarantee on a set of solutions in the sense that how far the CV error of the current best solution could be away from best possible CV error in the entire range of the regularization parameters. Our numerical experiments demonstrate that a theoretically guaranteed choice of a regularization parameter in the above sense is possible with reasonable computational costs.

Review:
A common task in machine learning is to learn the regularization parameter of a model. Usually this is done using cross-validation. The main result of the paper is that is presents a new way to analytically compute bounds on validation error. First, a lower and an upper bound for wx_i is computed for each validation instance x_i (w is the parameter of the model) as a function of the regularization parameter, given a solution for a different regularization parameter. Then, the scores for validation instances are combined to yield the bounds on the validation error. A central result in the paper is Lemma 1, that shows how compute the bounds for wx_i. The proof is not given in the paper, but the reader is referred to the Appendix. Giving intuition of this core result in the main text would be very important for the reader to be able to understand the idea without going to the Appendix, even if it's entirely reasonably to omit the full proof from the main text. The proof (in the appendix) seems to rely heavily on the convexity of the regularized loss function, limiting the usability (although being more general than presented before). Also, L2 prior on the parameter is used. Furthermore, the paper only deals with tuning a univariate hyper-parameter. The results appear correct, though I did not check the proofs in detail. The text is clearly written (though see the remark above). The experiments show that the regularization parameter can be optimized efficiently by assuming a reasonably tight error bound and, importantly, that the error bound is available in the first place. The topic seems relevant for the NIPS community, and the results interesting, though potentially too specific to raise wide interest.