Judea Pearl's insight that, when errors are assumed independent, the Pure (aka Natural) Direct Effect (PDE) is non-parametrically identified via the Mediation Formula was `path-breaking' in more than one sense! In the same paper Pearl described a thought-experiment as a way to motivate the PDE. Analysis of this experiment led Robins \& Richardson to a novel way of conceptualizing direct effects in terms of interventions on an expanded graph in which treatment is decomposed into multiple separable components. We further develop this novel theory here, showing that it provides a self-contained framework for discussing mediation without reference to cross-world (nested) counterfactuals or interventions on the mediator. The theory preserves the dictum `no causation without manipulation' and makes questions of mediation empirically testable in future Randomized Controlled Trials. Even so, we prove the interventionist and nested counterfactual approaches remain tightly coupled under a Non-Parametric Structural Equation Model except in the presence of a `recanting witness.' In fact, our analysis also leads to a simple sound and complete algorithm for determining identification in the (non-interventionist) theory of path-specific counterfactuals.
翻译:分析这一实验后,Robins ⁇ Richardson 找到了一种新颖的方法,将直接效果概念化,在扩大的图表中,治疗方法被分解成多种可分离的组成部分。我们在此进一步开发了这一新颖理论,表明它为讨论调解提供了一个自足的框架,而没有提及跨世界(被遗忘的)反事实或调解人的干预。我们的分析还保留了“未经操纵的无因果关系”的词句,并使调解问题在经验上可以在未来随机调整的控制审判中进行检验。即使如此,我们证明干预主义和嵌套的反事实方法仍然紧密地结合在非帕卡拉结构定量模型下,除非存在`被禁证人'。事实上,我们的分析还导致一种简单的正确和完全的判断方法。