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关注这篇论文的作者
Peter Bühlmann
Bin Yu

Bagging 的理论分析

研究问题
硬决策为何造成估计或分类的不稳定,bagging 如何降低这种不稳定性,以及基于子抽样的 subagging 能否减少计算成本?
方法与思路
作者形式化不稳定性,分析聚合对检验后估计和决策树等硬决策的平滑作用,并研究分段常数拟合中分割点的局部极限。
材料陈述的贡献
摘要称在所研究问题中刻画方差及均方误差下降,给出分割点的立方根速率极限结果,并论证 subagging 的计算优势。
阅读关注点
应核对不稳定性的定义、子样本规模条件、分割点局部邻域中的结论,以及从局部分析解释有限分割决策树的论证。
阅读边界
仅据版本未注明的索引摘要;性能结论针对特定硬决策问题,未提供具体条件与数值,不能认定所有学习器都受益。

原文摘要

Bagging is one of the most effective computationally intensive procedures to improve on unstable estimators or classifiers, useful especially for high dimensional data set problems. Here we formalize the notion of instability and derive theoretical results to analyze the variance reduction effect of bagging (or variants thereof) in mainly hard decision problems, which include estimation after testing in regression and decision trees for regression functions and classifiers. Hard decisions create instability, and bagging is shown to smooth such hard decisions, yielding smaller variance and mean squared error. With theoretical explanations, we motivate subagging based on subsampling as an alternative aggregation scheme. It is computationally cheaper but still shows approximately the same accuracy as bagging. Moreover, our theory reveals improvements in first order and in line with simulation studies. In particular, we obtain an asymptotic limiting distribution at the cube-root rate for the split point when fitting piecewise constant functions. Denoting sample size by n, it follows that in a cylindric neighborhood of diameter $n^{-1/3}$ of the theoretically optimal split point, the variance and mean squared error reduction of subagging can be characterized analytically. Because of the slow rate, our reasoning also provides an explanation on the global scale for the whole covariate space in a decision tree with finitely many splits.

笔记与分组