构建跨世界中介效应估计量的单世界混合形式

中文摘要 AI 辅助

在因果中介分析中，自然直接效应和自然间接效应通过嵌套反事实来定义，这些反事实将一种暴露水平下的结果与另一种暴露水平下的中介变量值相结合，因此本质上属于跨世界效应。其经典识别还依赖于跨世界独立性假设。因此，估计量本身及其识别假设均存在争议。本文探讨需要何种额外的单世界结构，才能将跨世界估计量重新表达为可在单世界假设下识别的单世界量。我们证明，在涉及易感性标记（一种可能潜在于基线的变量，编码了在参考处理组下中介变量的“将会是”的值）的假设下，这些量可以写成受控单世界效应的混合形式。若该标记可被观测，则标记限制的若干推论可进行经验检验。这些结果为跨世界中介效应提供了透明的单世界表述，同时明确了所需假设。尽管这些条件阐明了如何在单世界框架内解释跨世界估计量，但其实际相关性取决于此类易感性标记能否在具体应用中得到合理证明。我们讨论了其对主分层的影响，并在补充材料中展示了该表述如何扩展到有序中介变量以及存在暴露诱导的中介-结果混杂的设置。

英文摘要

In causal mediation analysis, natural direct and indirect effects are defined through nested counterfactuals that combine the outcome under one exposure level with the mediator value under another and are therefore inherently cross-world. Their canonical identification additionally relies on cross-world independence assumptions. Consequently, both the estimands and their identifying assumptions remain controversial. In this paper, we ask what additional single-world structure would be required to re-express cross-world estimands as single-world quantities, identifiable under single-world assumptions. We show that these quantities can be written as mixtures of controlled single-world effects under assumptions involving a susceptibility marker, a possibly latent baseline variable that encodes the "would-be" mediator value under a reference treatment arm. If observed, the marker would make several implications of the marker restrictions empirically testable. These results provide a transparent single-world formulation of cross-world mediation effects while making explicit the assumptions required. Although these conditions clarify how cross-world estimands can be interpreted within a single-world framework, their practical relevance depends on whether such susceptibility markers can be justified in specific applications. We discuss implications for principal stratification and show in the Supplementary Material how the formulation extends to ordered mediators and settings with exposure-induced mediator-outcome confounding.

发表机构

• Emory University（埃默里大学）

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