arXivDaily arXiv每日学术速递 周一至周五更新
arXiv 1507.02592cs.LGstat.ML
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统计和在线学习速度快

Fast rates in statistical and online learning

Universiteit Leiden(莱顿大学);Centrum voor Wiskunde en Informatica(数学与计算机科学中心);Australian National University(澳大利亚国立大学);NICTA(澳大利亚国家信息通信技术研究院)

Tim van Erven, Peter D. Grünwald, Nishant A. Mehta, Mark D. Reid, Robert C. Williamson

摘要:

机器学习的一个核心问题是学习算法在处理更多数据时的收敛速度--快速收敛意味着相同性能水平所需的数据更少。在线学习和统计学习中对快速学习的追求导致了学习理论中许多条件的发现,在这些条件下,快速学习是可能的。我们表明,这些条件中的大多数是一个单一的,统一的条件,有两种形式的特殊情况:“适当的”学习算法,总是输出一个假设在给定的模型,和随机混合性的在线算法,可能会作出预测的模型之外的中心条件。我们表明,在令人惊讶的弱假设下,这两个条件在某种意义上是等价的。中心条件有一个重新解释的凸性的一组伪概率,将其与密度估计下误指定。对于有界损失,我们展示了中心条件如何直接证明快速率,并证明了它与伯恩斯坦条件的等价性,伯恩斯坦条件本身就是Tsybakov边际条件的推广,两者都在统计学习中获得快速率方面发挥了核心作用。然而,虽然伯恩斯坦条件是双边的,但中心条件是单边的,这使得它更适合处理无限损失。在其随机混合性形式中,我们的条件推广了Juditsky,Rigollet和Tsybakov确定的随机指数条件和Vovk的混合性概念。因此,我们的统一条件提供了一个实质性的一步,在统计学习中的快速速率的表征,类似于经典的混性如何在连续的预测与专家的意见设置不断的遗憾。

英文摘要:

The speed with which a learning algorithm converges as it is presented with more data is a central problem in machine learning --- a fast rate of convergence means less data is needed for the same level of performance. The pursuit of fast rates in online and statistical learning has led to the discovery of many conditions in learning theory under which fast learning is possible. We show that most of these conditions are special cases of a single, unifying condition, that comes in two forms: the central condition for 'proper' learning algorithms that always output a hypothesis in the given model, and stochastic mixability for online algorithms that may make predictions outside of the model. We show that under surprisingly weak assumptions both conditions are, in a certain sense, equivalent. The central condition has a re-interpretation in terms of convexity of a set of pseudoprobabilities, linking it to density estimation under misspecification. For bounded losses, we show how the central condition enables a direct proof of fast rates and we prove its equivalence to the Bernstein condition, itself a generalization of the Tsybakov margin condition, both of which have played a central role in obtaining fast rates in statistical learning. Yet, while the Bernstein condition is two-sided, the central condition is one-sided, making it more suitable to deal with unbounded losses. In its stochastic mixability form, our condition generalizes both a stochastic exp-concavity condition identified by Juditsky, Rigollet and Tsybakov and Vovk's notion of mixability. Our unifying conditions thus provide a substantial step towards a characterization of fast rates in statistical learning, similar to how classical mixability characterizes constant regret in the sequential prediction with expert advice setting.

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