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Secondorder quantile methods for experts and combinatorial games.
 Proc. of the 28th Conf. on Learning Theory (COLT 2015), July 36,
, 2015
"... Abstract We aim to design strategies for sequential decision making that adjust to the difficulty of the learning problem. We study this question both in the setting of prediction with expert advice, and for more general combinatorial decision tasks. We are not satisfied with just guaranteeing mini ..."
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Abstract We aim to design strategies for sequential decision making that adjust to the difficulty of the learning problem. We study this question both in the setting of prediction with expert advice, and for more general combinatorial decision tasks. We are not satisfied with just guaranteeing minimax regret rates, but we want our algorithms to perform significantly better on easy data. Two popular ways to formalize such adaptivity are secondorder regret bounds and quantile bounds. The underlying notions of 'easy data', which may be paraphrased as "the learning problem has small variance" and "multiple decisions are useful", are synergetic. But even though there are sophisticated algorithms that exploit one of the two, no existing algorithm is able to adapt to both. The difficulty in combining the two notions lies in tuning a parameter called the learning rate, whose optimal value behaves nonmonotonically. We introduce a potential function for which (very surprisingly!) it is sufficient to simply put a prior on learning rates; an approach that does not work for any previous method. By choosing the right prior we construct efficient algorithms and show that they reap both benefits by proving the first bounds that are both secondorder and incorporate quantiles.
Fighting Bandits with a New Kind of Smoothness
"... We provide a new analysis framework for the adversarial multiarmed bandit problem. Using the notion of convex smoothing, we define a novel family of algorithms with minimax optimal regret guarantees. First, we show that regularization via the Tsallis entropy, which includes EXP3 as a special case, ..."
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We provide a new analysis framework for the adversarial multiarmed bandit problem. Using the notion of convex smoothing, we define a novel family of algorithms with minimax optimal regret guarantees. First, we show that regularization via the Tsallis entropy, which includes EXP3 as a special case, matches the O( NT) minimax regret with a smaller constant factor. Second, we show that a wide class of perturbation methods achieve a nearoptimal regret as low as O( NT logN), as long as the perturbation distribution has a bounded hazard function. For example, the Gumbel, Weibull, Frechet, Pareto, and Gamma distributions all satisfy this key property and lead to nearoptimal algorithms. 1