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arXiv 0908.3458stat.MLmath.STstat.TH
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最优无偏值估计及其与LSTD、TD和MC的关系

The Optimal Unbiased Value Estimator and its Relation to LSTD, TD and MC

Steffen Grünewälder, Klaus Obermayer

摘要:

在这项分析研究中,我们推导出最佳无偏值估计(MVU),并比较其统计风险的三个著名的值估计:时间差分学习(TD),蒙特卡罗估计(MC)和最小二乘时间差分学习(LSTD)。我们证明,LSTD是等价的MVU,如果马尔可夫奖励过程(MRP)是非循环的,并表明,这两个不同的最循环的MRP作为LSTD,然后通常偏置。更一般地说,我们表明,估计,满足贝尔曼方程只能是无偏的特殊循环MRP。主要原因是采取预期的概率测度。这些措施不同,从状态到状态,并由于强烈的耦合,由贝尔曼方程,它通常是不可能的一组值估计是无偏的,相对于这些措施。此外,我们推导出的MVU MC和TD的关系。最重要的是MC与MVU和未折扣MRP的LSTD的等效性,其中MC具有相同的信息量。在贴现的情况下,这种等价性不再成立。对于TD,我们表明,它基本上是无偏的非循环MRPs和有偏的循环MRPs。我们还根据风险对估计量进行排序,并给出反例表明MVU和LSTD之间、MC和LSTD之间以及TD和MC之间不存在一般的排序。理论结果支持的例子和实证评价。

英文摘要:

In this analytical study we derive the optimal unbiased value estimator (MVU) and compare its statistical risk to three well known value estimators: Temporal Difference learning (TD), Monte Carlo estimation (MC) and Least-Squares Temporal Difference Learning (LSTD). We demonstrate that LSTD is equivalent to the MVU if the Markov Reward Process (MRP) is acyclic and show that both differ for most cyclic MRPs as LSTD is then typically biased. More generally, we show that estimators that fulfill the Bellman equation can only be unbiased for special cyclic MRPs. The main reason being the probability measures with which the expectations are taken. These measure vary from state to state and due to the strong coupling by the Bellman equation it is typically not possible for a set of value estimators to be unbiased with respect to each of these measures. Furthermore, we derive relations of the MVU to MC and TD. The most important one being the equivalence of MC to the MVU and to LSTD for undiscounted MRPs in which MC has the same amount of information. In the discounted case this equivalence does not hold anymore. For TD we show that it is essentially unbiased for acyclic MRPs and biased for cyclic MRPs. We also order estimators according to their risk and present counter-examples to show that no general ordering exists between the MVU and LSTD, between MC and LSTD and between TD and MC. Theoretical results are supported by examples and an empirical evaluation.

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