提出理想启发式估计器应具备自校正能力,避免预测自身误差。
Towards a Law of Iterated Expectations for Heuristic Estimators
- 设计满足迭代估计与误差正交性的启发式估计框架
- 证明在两类问题中准确率无法实现零均值误差
- 为理解神经网络行为提供新思路,适合理论研究者
Christiano等人(2022)将启发式估计器定义为一种假设性算法,用于从论证中估算数学表达式的值。本文主张:理想启发式估计器不应能预测自身误差,并探讨该原则的形式化路径。最直接的表述是,对任意表达式 $Y$ 与论证 $π$,有 $\mathbb{G}(Y - \mathbb{G}(Y \mid π) \mid π) = 0$。我们进一步提出两个更强性质:迭代估计(类比迭代期望律)与误差正交性。尽管直观合理,验证这些性质仍具挑战。作为替代,我们考察‘准确性’——即估计器在数学表达式分布上的平均误差为零。然而,在两个具体估计问题中,我们揭示了构造此类准确估计器的障碍。最后讨论未来方向及启发式估计器在理解神经网络行为中的潜在应用。
原文摘要 · Abstract (English)
Christiano et al. (2022) define a *heuristic estimator* to be a hypothetical algorithm that estimates the values of mathematical expressions from arguments. In brief, a heuristic estimator $\mathbb{G}$ takes as input a mathematical expression $Y$ and a formal "heuristic argument" $π$, and outputs an estimate $\mathbb{G}(Y \mid π)$ of $Y$. In this work, we argue for the informal principle that a heuristic estimator ought not to be able to predict its own errors, and we explore approaches to formalizing this principle. Most simply, the principle suggests that $\mathbb{G}(Y - \mathbb{G}(Y \mid π) \mid π)$ ought to equal zero for all $Y$ and $π$. We argue that an ideal heuristic estimator ought to satisfy two stronger properties in this vein, which we term *iterated estimation* (by analogy to the law of iterated expectations) and *error orthogonality*. Although iterated estimation and error orthogonality are intuitively appealing, it can be difficult to determine whether a given heuristic estimator satisfies the properties. As an alternative approach, we explore *accuracy*: a property that (roughly) states that $\mathbb{G}$ has zero average error over a distribution of mathematical expressions. However, in the context of two estimation problems, we demonstrate barriers to creating an accurate heuristic estimator. We finish by discussing challenges and potential paths forward for finding a heuristic estimator that accords with our intuitive understanding of how such an estimator ought to behave, as well as the potential applications of heuristic estimators to understanding the behavior of neural networks.
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